Rename the -cc1 option "-generate-module-index" to
"-fmodules-global-index" and expand its behavior to include both the
use and generation of the global module index.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173404 91177308-0d34-0410-b5e6-96231b3b80d8