swift234

Add a hack to the ASTPrinter, when printing PatternBindingDecls to print 
immutable variables with a let prefix instead of a 'var' prefix.  This allows
us to AST print this as 'let' instead of 'var', which isn't totally awesome,
but is better than misleading people with var.

func f() {
  let x : Int  = 42
}

Also, fix the SILPrinter to explode pattern binding decls, so that it doesn't
fall into the booby traps inherent in PatternBindingDecl printing.


Swift SVN r15393