[PCH] When deserializing an IdentifierInfo, call IdentifierInfo::RevertTokenIDToIdentifier() only when it's not already an identifier.

Fixes an assertion hit.
rdar://13288735

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176148 91177308-0d34-0410-b5e6-96231b3b80d8