[modules] Attempt to improve performance for declaration merging without a Sema

object in C. Rather than using the DeclContext (which is very slow because it
triggers us to build a lookup table for the DeclContext), use a separate map
from identifiers to decls for this case, containing only the ones we've
actually deserialized. For convenience, this map is implemented as an
IdentifierResolver.

llvm-svn: 268817