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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268817 91177308-0d34-0410-b5e6-96231b3b80d8