Allow getting all source locations of selector identifiers in a ObjCMessageExpr.

Instead of always storing all source locations for the selector identifiers
we check whether all the identifiers are in a "standard" position; "standard" position is

  -Immediately before the arguments: [foo first:1 second:2]
  -With a space between the arguments: [foo first: 1 second: 2]
  -For nullary selectors, immediately before ']': [foo release]

In such cases we infer the locations instead of storing them.

llvm-svn: 140987