Module debugging: Fix the DWO filename for PCH in a relative path.

PCH in a relative location had a redundant relative path on the DWO
filename and the DW_AT_compilation_dir.
This patch fixes this and adds testcases for PCH in the same directory,
in a relative, and an absolute location.

rdar://problem/25537947

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267740 91177308-0d34-0410-b5e6-96231b3b80d8