[ExpressionParser] Test GetClangResourceDir

Summary:
I'm doing this because I plan on implementing `ComputeClangResourceDirectory`
on windows so that `GetClangResourceDir` will work.  Additionally, I made
test_paths make sure that the directory member of the returned FileSpec is not
none. This will fail on windows since `ComputeClangResourceDirectory` isn't
implemented yet.

Differential Revision: https://reviews.llvm.org/D58748

llvm-svn: 355463