swift234

Warn and fix-it if second _compiler_version component isn't a *

To better indicate that the second _compiler_version component isn't
used in the ordering, warn and provide a fix-it replacement if it's
not a '*' in source code.

To make the diagnostics a little easier to emit, I cleaned up the
parsing code to use StringRef::split instead of a hand-written
tokenizer.

rdar://problem/23080845

Swift SVN r32673