Deduplicate class export and fix identifier error in ExportNameScopeToCpp (#7550)

Added method_alias.carbon test so that the class export code in
`ExportNameScopeToCpp` is tested. Refactored `ExportClassToCpp` so that
`ExportNameScopeToCpp` can reuse that code.

Moved the `identifier_info` code in `ExportNameScopeToCpp` into the
namespace block, because the name scope's name ID is not valid for
classes.

Added a call to `CompleteType` for classes exported via
`ExportNameScopeToCpp`, otherwise a "queried property of class with no
definition" assert is later reached (when adding methods) in the call
chain `BuildCppToCarbonThunkDecl` -> `DeclContext::addHiddenDecl` ->
`CXXRecordDecl::addedMember` -> `CXXRecordDecl::data`.

toolchain/check/testdata/interop/cpp/class/export/method.carbon

@@ -88,3 +88,17 @@ void F() {
   Carbon::C().M();
 }
 ''';
+
+// --- method_alias.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class A {
+  fn F();
+}
+
+alias G = A.F;
+
+inline Cpp '''
+void call() { Carbon::G(); }
+''';