add LLVM_VERSION_MAJOR and _MINOR defines

This is useful for clients that want to maintain compatibility
across multiple releases of LLVM. Currently users like Klee and
Mesa all have to roll their own 'parse llvm-config --version
output and generate defines' solution.

Also reuse the new macros so that version information is less
redundant/likely to fall out of sync again in the future.

llvm-svn: 150405