Improve coordination between the module manager and the global module
index, optimizing the operation that skips lookup in modules where we
know the identifier will not be found. This makes the global module
index optimization actually useful, providing an 8.5% speedup over
modules without the global module index for -fsyntax-only.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173529 91177308-0d34-0410-b5e6-96231b3b80d8