[lld] Deprecate using llvm-config to detect llvm installation

This is continuing in the path of D51714, which did this for Clang.

I have rearranged the source code Clang so one can diff the top-level
CMakeLists.txt of Clang and LLD, ensuring we use the same strategy for
both.

Besides diffing the two files, `git diff --color-moved` on LLD also helps review.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D116492