CMake's find_package produces full paths even if we could rely on the
linker to find the library in the standard search path. In that case
perhaps llvm-config --system-libs should reflect this and also use
full paths to mirror the link command that we're using.
This could be an issue for distributors, if they build with libraries in
some local path, but expect users to have them in their standard search
Fixes an issue raised in D132799, since we're now using the same list
for linking libLLVM.
As an alternative, we could produce two different lists, one with just
stems for llvm-config and another with (possibly) full paths for