[llvm-rc] Handle driveless absolute windows paths when loading external files
When llvm-rc loads an external file, it looks for it relative to
a number of include directories and the current working directory.
If the path is considered absolute, llvm-rc tries to open the
filename as such, and doesn't try to open it relative to other
On Windows, a path name like "\dir\file" isn't considered absolute
as it lacks the drive name, but by appending it on top of the search
dirs, it's not found.
LLVM's sys::path::append just appends such a path (same with a properly
absolute posix path) after the paths it's supposed to be relative to.
This fix doesn't handle the case if the resource script and the
external file are on a different drive than the current working
directory; to fix that, we'd have to make LLVM's sys::path::append
handle appending fully absolute and partially absolute paths (ones
lacking a drive prefix but containing a root directory), or switch
to C++17's std::filesystem.
Differential Revision: https://reviews.llvm.org/D92558