Make it easier to build against a pre-built Clang package with CMake

Installing <prefix>/share/clang/cmake/ClangConfig.cmake makes CMake's
builtin find_package() utility work with Clang. This also allows
downstream consumers of Clang to statically link against libraries like
clangAST and have that pull in dependencies like clangBasic and
LLVMSupport.

See the CMake docs on packages:
http://www.cmake.org/cmake/help/v3.0/manual/cmake-packages.7.html