PCH debug info: Avoid appending the source directory to an absolute path

When building a precompiled header in -fmodule-format=obj (i.e.,
`-gmodules) in an absolute path, the locig in
CGDebugInfo::createCompileUnit would unconditionally append the source
directory to the -main-file-name. This patch avoids that behavior for
absolute paths.

rdar://problem/46045865

Differential Revision: https://reviews.llvm.org/D69213

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375423 91177308-0d34-0410-b5e6-96231b3b80d8