When determining the install prefix in LLVMConfig.cmake etc resolve
symlinks in CMAKE_CURRENT_LIST_FILE first. The motivation for this is to
support symlinks like `/usr/lib64/cmake/llvm` to
`/usr/lib64/llvm19/lib/cmake/llvm`. This only works correctly if the
paths are relative to the resolved symlink.
It's worth noting that this *mostly* already works out of the box,
because cmake automatically does the symlink resolution when the library
is found via CMAKE_PREFIX_PATH. It just doesn't happen when it's found
via the default prefix path.
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