Right now the external_source_symbol attribute isn't supported by #pragma clang attribute for the following two reasons:
- The Named attribute subject isn't supported by TableGen.
- There was no way to specify a subject match rule for #pragma clang attribute that could operate on a set of attribute subjects (e.g. the ones that derive from NamedDecl).
This patch fixes the two issues and thus adds external_source_symbol support to #pragma clang attribute,