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/import/constexpr.carbon

@@ -0,0 +1,61 @@
+// 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/full.carbon
+// EXTRA-ARGS: --clang-arg=--std=c++20
+//
+// 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/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/import/constexpr.carbon
+
+// --- bool.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+constexpr bool b = true;
+''';
+
+class C(B:! bool) {}
+fn F() -> C(Cpp.b);
+let x: C(true) = F();
+
+// --- int.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+constexpr int i = 123;
+''';
+
+class C(I:! i32) {}
+fn F() -> C(Cpp.i);
+let x: C(123) = F();
+
+// --- float.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+constexpr float flt = 123.5;
+''';
+
+class C(V:! f32) {}
+fn F() -> C(Cpp.flt);
+let x: C(123.5) = F();
+
+// --- pointer.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+int i = 123;
+constexpr int* _Nonnull ptr = &i;
+''';
+
+class C(V:! i32*) {}
+fn F() -> C(Cpp.ptr);
+let x: C(&Cpp.i) = F();