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/cpp/generate_ast.cpp

@@ -435,6 +435,10 @@ auto CarbonExternalASTSource::MapInstIdToClangDeclOrType(LookupResult lookup)
     case CARBON_KIND(SemIR::FieldDecl field_decl): {
       return ExportFieldToCpp(*context_, target_inst_id, field_decl);
     }
+    case CARBON_KIND(SemIR::VarStorage var_storage): {
+      return ExportVarToCpp(*context_, SemIR::LocId(target_inst_id),
+                            var_storage);
+    }
     default:
       return nullptr;
   }