[llvm-config] Add --cmakedir to obtain CMake module location

Add a --cmakedir option to llvm-config that returns the correct path to
built/installed CMake modules (i.e. lib/cmake/llvm). This is mostly
intended as a convenience option for stand-alone builds of other LLVM
projects that frequently reconstruct LLVM_CMAKE_PATH after querying
llvm-config.

Differential Revision: https://reviews.llvm.org/D26894

llvm-svn: 291218