Modify the pragma handlers to accept and use StringRefs instead of IdentifierInfos.

When loading the PCH, IdentifierInfos that are associated with pragmas cause declarations that use these identifiers to be deserialized (e.g. the "clang" pragma causes the "clang" namespace to be loaded).
We can avoid this if we just use StringRefs for the pragmas.

As a bonus, since we don't have to create and pass IdentifierInfos, the pragma interfaces get a bit more simplified.

llvm-svn: 108237