function into all of the expression nodes. Re-implement
Expr::getStartLoc() in terms of this function, and add it's brother
Expr::getEndLoc(), removing the specialized implementations.
Clean up the source ranges of implicitly-created tuple expressions in
the process.
Swift SVN r855
distinct from the SMXXX types. This is important because SMRange and
SourceRange have subtly different semantics, and is also nice to isolate
SMLoc from swift.
Swift SVN r749
