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, 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::".