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.

This was in CMake syntax generation, so we didn't catch it eval time.

Follow up from D117973

Firstly, we we make an additional GNUInstallDirs-style variable. With
NixOS, for example, this is crucial as we want those to go in
`${dev}/lib/cmake` not `${out}/lib/cmake` as that would a cmake subdir
of the "regular" libdir, which is installed even when no one needs to do
any development.

Secondly, we make *Config.cmake robust to absolute package install
paths. We for NixOS will in fact be passing them absolute paths to make
the `${dev}` vs `${out}` distinction mentioned above, and the
GNUInstallDirs-style variables are suposed to support absolute paths in
general so it's good practice besides the NixOS use-case.

Thirdly, we make `${project}_INSTALL_PACKAGE_DIR` CACHE PATHs like other
install dirs are.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D117973

See the docs in the new function for details.

 I think I found every instance of this copy pasted code. Polly could
 also use it, but currently does something different, so I will save the
 behavior change for a future revision.

We get the shared, non-installed CMake modules following the pattern
established in D116472.

It might be good to have LLD and Flang also use this, but that would be
a functional change and so I leave it as future work.

Reviewed By: beanz, lebedev.ri

Differential Revision: https://reviews.llvm.org/D116521