[cmake] Obtain LLVM_CMAKE_PATH from llvm-config if available

Use the new --cmakedir option to obtain LLVM_CMAKE_PATH straight from
llvm-config. Fallback to local reconstruction if llvm-config does not
support this option.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@291506 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed