[Remarks] Add support for prepending a path to external files

This helps with testing and debugging for paths that are assumed
absolute.

It also uses a FileError to provide the file path it's trying to open.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375008 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed