Implement bracket insertion for Objective-C instance message sends as

part of parser recovery. For example, given:

  a method1:arg];

we detect after parsing the expression "a" that we have the start of a
message send expression. We pretend we've seen a '[' prior to the a,
then parse the remainder as a message send. We'll then give a
diagnostic+fix-it such as:

fixit-objc-message.m:17:3: error: missing '[' at start of message
      send expression
  a method1:arg];
  ^
  [

The algorithm here is very simple, and always assumes that the open
bracket goes at the beginning of the message send. It also only works
for non-super instance message sends at this time.

llvm-svn: 113968