Sink SpecifierInfo into the only class that uses it.

SpecifierInfo is not used outside of NamespaceSpecifierSet except
indirectly through NamespaceSpecifierSet's iterator, so clean up the
code a bit by moving SpecifierInfo into NamespaceSpecifierSet. Also drop
SpecifierInfo's trivial yet verbose constructor since brace
initiialization is sufficient in the only two places the constructor was
being explicitly called.

No functionality changed.

llvm-svn: 210672