Replace obsolete LLVM_CONFIG_PATH

Replace the obsolete LLVM_CONFIG_PATH uses with LLVM_CMAKE_DIR in case
of compiler-rt, or remove it entirely in case of libcxxabi (as it was
removed a while ago, so the statement was not doing naything). The last
use of LLVM_CONFIG_PATH was deprecated in D137024.

Differential Revision: https://reviews.llvm.org/D137226