Require a module.map file to load a module

Removes some old code that allowed a module to be loaded from a pcm file
even if the module.map could not be found.  Also update a number of
tests that relied on the old behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199852 91177308-0d34-0410-b5e6-96231b3b80d8