Expose LLVM_VERSION_PATCH in llvm-config.h

There was no reason to keep this private in config.h, and users
requested that it be available in PR22615.

Also fix a bug where patch versions of '0' would cause the macro to
remain undefined. The "#cmakedefine" command only creates a macro if the
named variable would be considered true in the context of an if().

llvm-svn: 229529