Module Debugging: Canonicalize the file names used as PCH module names
by stripping the path. Follow-up to r258555.

This is safe because only one PCH per CU is currently supported for
module debugging.

rdar://problem/24301262

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@258582 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit f5a6cecb8da7522684057bdf1029c01317029664)
4 files changed