clang/cmake: Drop use of llvm-config for LLVM install discovery

This has been deprecated for a while, since D51714 in 2018.

Remove it in favor of using CMake's find_package() function.

Reviewed By: phosek, mgorny

Differential Revision: https://reviews.llvm.org/D128777

GitOrigin-RevId: d2b158e29eedf4a29bf8d2142f2ed21a52fc80a7
1 file changed