Alter the ExternalASTSource interface to permit by-name lookups. PCH continues to

bring in the entire lookup table at once.

Also, give ExternalSemaSource's vtable a home.  This is important because otherwise
any reference to it will cause RTTI to be emitted, and since clang is compiled
with -fno-rtti, that RTTI will contain unresolved references (to ExternalASTSource's
RTTI).  So this change makes it possible to subclass ExternalSemaSource from projects
compiled with RTTI, as long as the subclass's home is compiled with -fno-rtti.

llvm-svn: 105268