[Lex] inferModuleFromLocation should do no work if there are no modules

getModuleContainingLocation ends up on the hot-path for typical C code
which can lead to calls to getFileIDSlow.

To speed this up, short circuit inferModuleFromLocation when there
aren't any modules, implicit or otherwise.

This shaves 4-5% build time when building the linux kernel.

llvm-svn: 269687