Add an extension point to the CommandLine library where clients can

Add an extension point to the CommandLine library where clients can
register extra version information to be printed. This is designed to
allow those tools which link in various targets to also print those
registered targets under --version.

Currently this printing logic is embedded into the Support library
directly; a huge layering violation. This is the first step to hoisting
it out into the tools without adding lots of duplicated code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135755 91177308-0d34-0410-b5e6-96231b3b80d8