swift234

Fix <rdar://problem/20364082> Incorrect fixit "if let x: T = ..." => "if let x?: T = ..."

Previously we would emit one diagnostic for the "you need a ?" and one for the "you can't use
a type annotation" errors.  This doesn't work with Xcode because if you apply one, the other
gets clobbered.  Merge these into one diagnostic that performs the removal and the insert.



Swift SVN r26902