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

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: -(id)first:(int)x second:(int)y;
  -With a space between the arguments: -(id)first: (int)x second: (int)y;
  -For nullary selectors, immediately before ';': -(void)release;

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140989 91177308-0d34-0410-b5e6-96231b3b80d8