Enable the global module index by default. Introduce the
-fno-modules-global-index -cc1 option to allow one to disable the
index for performance testing purposes, but with a 10% win in
-fsyntax-only time, there is no reason a user would do this.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173707 91177308-0d34-0410-b5e6-96231b3b80d8