swift234

rework our treatment of identifiers in refutable patterns that are inside of
a let/var pattern.  Now any identifier in one of these is a variable binding,
not sometimes a value references (depending on contextual syntax).

This isn't expected to have a widespread effect on existing real world code:
 - No impact on the stdlib.
 - It does fix two validation crash tests, but possibly because the original issue is hidden by a different diagnostic path in the compiler.
 - This needed two tests to be tweaked to undistribute "let".

On the positive side, this means that "case let x?:" now works properly, woo.



Swift SVN r26000