Clean "./" from __FILE__ expansion.

This is alternative to https://reviews.llvm.org/D121733
and helps with Clang header modules in which FILE
may expand to "./foo.h" or "foo.h" depending on whether the file was
included directly or not.

Only do this when UseTargetPathSeparator is true, as we are already
changing the path in that case.

Reviewed By: ayzhao

Differential Revision: https://reviews.llvm.org/D126396