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
llvm-svn: 375423
Adrian Prantl