[TextAPI] Introduce SymbolSet

SymbolSet is a structure that acts as a simple container class for exported symbols that
belong to a library interface. It allows tapi to decouple the globals
from the other library attributes. It's uniqued by symbol name and `kind`, which all contain their assigned target triples.

Reviewed By: zixuw

Differential Revision: https://reviews.llvm.org/D149860