Support import Cpp; to import Cpp namespace for using C++ builtins (#6320)

This allows writing
```
import Cpp;
```

Instead of writing
```
import Cpp inline "";
```

Part of #6330.

toolchain/check/testdata/interop/cpp/function/void_pointer.carbon

@@ -88,7 +88,7 @@ fn F() {
 
 library "[[@TEST_NAME]]";
 
-import Cpp inline "";
+import Cpp;
 
 // CHECK:STDERR: fail_void.carbon:[[@LINE+5]]:6: error: parameter has incomplete type `Cpp.void` in function definition [IncompleteTypeInFunctionParam]
 // CHECK:STDERR: fn F(x: Cpp.void) {}