Export Carbon global variables and static vars to C++ (#7298)

Add `ExportVarToCpp`. This checks the `clang_decls` mapping and returns
an existing decl if found. Otherwise, it creates a new `VarDecl` and
adds it to the `clang_decls` mapping.

When lowering, in `FileContext::BuildGlobalVariableDecl`, the
`clang_decls` mapping is used to lookup an existing
`llvm::GlobalVariable` for the instruction. If found, use that rather
than creating a new one to avoid an unwanted second definition in the
llvm IR.

toolchain/check/testdata/interop/cpp/class/export/static.carbon

@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/class/export/static.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/export/static.carbon
+
+// --- static.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class C {
+  static var x: i32 = 123;
+}
+
+inline Cpp '''
+int F() {
+  return Carbon::C::x;
+}
+''';

toolchain/check/testdata/interop/cpp/primitive_types/array.carbon

@@ -30,7 +30,7 @@ fn G(x: Cpp.X, n: i32) -> i32 {
   return x.arr[n];
 }
 
-// --- fail_todo_export_variable.carbon
+// --- export_variable.carbon
 library "[[@TEST_NAME]]";
 
 import Cpp;
@@ -39,10 +39,6 @@ var arr: array(i32, 5);
 
 inline Cpp '''
 int F(int n) {
-  // CHECK:STDERR: fail_todo_export_variable.carbon:[[@LINE+4]]:18: error: no member named 'arr' in namespace 'Carbon' [CppInteropParseError]
-  // CHECK:STDERR:    13 |   return Carbon::arr[n];
-  // CHECK:STDERR:       |                  ^~~
-  // CHECK:STDERR:
   return Carbon::arr[n];
 }
 ''';

toolchain/check/testdata/interop/cpp/var/export/var.carbon

@@ -0,0 +1,45 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/export/var.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/export/var.carbon
+
+// --- var.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+var x: i32 = 123;
+
+inline Cpp '''
+int Get() {
+  return Carbon::x;
+}
+''';
+
+// --- fail_var_unsupported_type.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+// CHECK:STDERR: fail_var_unsupported_type.carbon:[[@LINE+4]]:5: error: semantics TODO: `failed to map Carbon type to C++` [SemanticsTodo]
+// CHECK:STDERR: var x: () = ();
+// CHECK:STDERR:     ^~~~~
+// CHECK:STDERR:
+var x: () = ();
+
+inline Cpp '''
+void F() {
+  // CHECK:STDERR: fail_var_unsupported_type.carbon:[[@LINE+4]]:11: error: no member named 'x' in namespace 'Carbon' [CppInteropParseError]
+  // CHECK:STDERR:    17 |   Carbon::x;
+  // CHECK:STDERR:       |           ^
+  // CHECK:STDERR:
+  Carbon::x;
+}
+''';

toolchain/check/testdata/interop/cpp/var/import/constexpr.carbon

@@ -7,9 +7,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/constexpr.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/import/constexpr.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/constexpr.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/import/constexpr.carbon
 
 // --- bool.carbon
 

toolchain/check/testdata/interop/cpp/var/import/global.carbon

@@ -7,9 +7,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/global.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/import/global.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/global.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/import/global.carbon
 
 // ============================================================================
 // Global scope

toolchain/check/testdata/interop/cpp/var/import/namespace.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/namespace.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/var/import/namespace.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/namespace.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/var/import/namespace.carbon
 
 // --- namespace.h