Add support for a patch version to the cmake system.

Patch by Brad King

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206425 91177308-0d34-0410-b5e6-96231b3b80d8
Teach LLVMConfigVersion.cmake to behave as find_package() expects.

Patch by Brad King

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206426 91177308-0d34-0410-b5e6-96231b3b80d8