Expose LLVM version string via macro in llvm-config.h, and modify Go bindings
to make use of it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222307 91177308-0d34-0410-b5e6-96231b3b80d8
Teach llvm-build to avoid touching LibraryDependencies.inc unless the contents
change. This saves us from rebuilding llvm-config each time we reconfigure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222308 91177308-0d34-0410-b5e6-96231b3b80d8