[Modules] Use global index to improve typo correction performance

Typo correction for an unqualified name needs to walk through all of the identifier tables of all modules.
When we have a global index, just walk its identifier table only.

rdar://13425732

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179730 91177308-0d34-0410-b5e6-96231b3b80d8