Remove "__cpp_thunk" from thunk name for exported functions (#7092)

This makes the behavior consistent between methods and regular
functions, and fixes the behavior with `using` aliases (see new
`using.carbon` test).

As requested in
https://github.com/carbon-language/carbon-lang/pull/7078#discussion_r3121104954.

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

@@ -57,6 +57,19 @@ void G() {
 }
 ''';
 
+// --- using.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn F();
+
+inline Cpp '''
+using Carbon::F;
+void G() { F(); }
+''';
+
 // --- fail_todo_generic.carbon
 
 library "[[@TEST_NAME]]";