Add unit tagging to CppGlobalVarId (#6263)

toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon

@@ -61,7 +61,7 @@ fn G(x: Cpp.X) {
 // CHECK:STDOUT:     entity_name60000002: {name: name00000002, parent_scope: name_scope<none>, index: -1, is_template: 0}
 // CHECK:STDOUT:     entity_name60000003: {name: name00000005, parent_scope: name_scope60000001, index: -1, is_template: 0}
 // CHECK:STDOUT:   cpp_global_vars:
-// CHECK:STDOUT:     cpp_global_var00000000: {key: {entity_name_id: entity_name60000003}, clang_decl_id: clang_decl_id00000007}
+// CHECK:STDOUT:     cpp_global_var60000000: {key: {entity_name_id: entity_name60000003}, clang_decl_id: clang_decl_id00000007}
 // CHECK:STDOUT:   functions:
 // CHECK:STDOUT:     function60000000: {name: name00000001, parent_scope: name_scope00000000, call_params_id: inst_block60000006, body: [inst_block60000009]}
 // CHECK:STDOUT:     function60000001: {name: name00000004, parent_scope: name_scope60000001, call_params_id: inst_block_empty}

toolchain/sem_ir/file.cpp

@@ -36,6 +36,7 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       value_stores_(&value_stores),
       filename_(std::move(filename)),
       entity_names_(check_ir_id),
+      cpp_global_vars_(check_ir_id),
       functions_(check_ir_id),
       cpp_overload_sets_(check_ir_id),
       classes_(check_ir_id),