[cmake] Resolve symlink when finding install prefix (llvm#124743)

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.

Author: nikic

cmake/Modules/FindPrefixFromConfig.cmake

@@ -39,10 +39,10 @@ function(find_prefix_from_config out_var prefix_var path_to_leave)
     # install prefix, and avoid hard-coding any absolute paths.
     set(config_code
       "# Compute the installation prefix from this LLVMConfig.cmake file location."
-      "get_filename_component(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
+      "get_filename_component(${prefix_var} \"\${CMAKE_CURRENT_LIST_FILE}\" REALPATH)")
     # Construct the proper number of get_filename_component(... PATH)
     # calls to compute the installation prefix.
-    string(REGEX REPLACE "/" ";" _count "${path_to_leave}")
+    string(REGEX REPLACE "/" ";" _count "${path_to_leave}/plus_one")
     foreach(p ${_count})
       list(APPEND config_code
         "get_filename_component(${prefix_var} \"\${${prefix_var}}\" PATH)")