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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269687 91177308-0d34-0410-b5e6-96231b3b80d8