[llvm-rc] Support absolute filenames in manifests

CMake generate manifests that contain absolute filenames and these
currently result in assertion error. This change ensures that we
handle these correctly.

Differential Revision: https://reviews.llvm.org/D54194

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346450 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed