[AST/etc] Mark {getSourceRange(),getStartLoc(),getEndLoc()} as LLVM_READONLY.

 - The theory here is that we have these functions sprinkled in all over the
   place. This should allow the optimizer to at least realize it can still do
   load CSE across these calls.
 - I blindly marked all instances as such, even though the optimizer can infer
   this attribute in some instances (some of the inline ones) as that was easier
   and also, when given the choice between thinking and not thinking, I prefer
   the latter.

You might think this is mere frivolity, but actually this is good for a .7 -
1.1% speedup on 403.gcc/combine.c, JSC/Interpreter.cpp,
OGF/NSBezierPath-OAExtensions.m.

llvm-svn: 152426