[AST] Reimplement Stmt::getLoc{Start,End} to dispatch to subclass overloads.

 - getSourceRange() can be very expensive, we should try to avoid it if at all possible.

In conjunction with the previous commit I measured a ~2% speedup on 403.gcc/combine.c and a 3% speedup on OmniGroupFrameworks/NSBezierPath-OAExtensions.m.

llvm-svn: 152411