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/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;
+}
+''';