Move VersionTuple from clang/Basic to llvm/Support

Summary:
This kind of functionality is useful to other project apart from clang.
LLDB works with version numbers a lot, but it does not have a convenient
abstraction for this. Moving this class to a lower level library allows
it to be freely used within LLDB.

Since this class is used in a lot of places in clang, and it used to be
in the clang namespace, it seemed appropriate to add it to the list of
adopted classes in LLVM.h to avoid prefixing all uses with "llvm::".

Also, I didn't find any tests specific for this class, so I wrote a
couple of quick ones for the more interesting bits of functionality.

Reviewers: zturner, erik.pilkington

Subscribers: mgorny, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D47887

llvm-svn: 334399