Factor the trait for lookup into the on-based hash table of

Factor the trait for lookup into the on-based hash table of
identifiers into two parts: the part that involves dealing with the
key (which can be re-used) and the ASTReader-specific part that
creates the IdentifierInfos. While I'm at it, StringRef'ify this code,
which was using pair<const char*, unsigned>. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173283 91177308-0d34-0410-b5e6-96231b3b80d8