[cmake] cleanup on clang version mapping on macOS

This patch cleans up the code that determines compiler type and version
on macOS for Xcode-based LLVM/CLANG.  As it turns out, there is a well
documented mapping for clang versions: see [1] and [2].

I also updated unified regex patterns used to find versions of the
compiler in its version string.

References:
  [1] https://trac.macports.org/wiki/XcodeVersionInfo
  [2] https://en.wikipedia.org/wiki/Xcode#Version_history

Change-Id: I656dd4b20c7c852746861861f0741763b02609d5
Reviewed-on: http://gerrit.cloudera.org:8080/18874
Tested-by: Kudu Jenkins
Reviewed-by: Attila Bukor <abukor@apache.org>
1 file changed