Add #pragma clang attribute support to the external_source_symbol attribute

Prior to this commit the external_source_symbol attribute wasn't supported by
#pragma clang attribute for the following two reasons:

- The Named attribute subject hasn't been 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 commit fixes the two issues and thus adds external_source_symbol support to
#pragma clang attribute.

rdar://31169028

Differential Revision: https://reviews.llvm.org/D32176

llvm-svn: 300712