Make clangd CompletionModel not depend on directory layout.

Make clangd CompletionModel not depend on directory layout.

The current code accounts for two possible layouts, but there is at
least a third supported layout: clang-tools-extra may also be checked
out as clang/tools/extra with the releases, which was not yet handled.
Rather than treating that as a special case, use the location of
CompletionModel.cmake to handle all three cases. This should address the
problems that prompted D96787 and the problems that prompted the
proposed revert D100625.

Reviewed By: usaxena95

Differential Revision: https://reviews.llvm.org/D101851