swift234

Don't look through 'try!' in getSemanticsProvidingExpr().

To support this, make 'try' and 'try!' no longer IdentityExprs
and give them a common base class to simplify the sorts of
analyses and transformations that do want to treat them
as identity-like.

Note that getSPE() still looks through normal 'try', since
the overwhelming proportion of clients will consider it
semantically equivalent to the undecorated expression.

Change getValueProvidingExpr() to look through try!, since
it's allowed to return something with slightly different
semantics, and use it in the unused-result diagnostic.

Fixes a large number of bugs, mostly uncaught, with SILGen
peepholes that use getSPE() and therefore were accidentally
looking through try!.  <rdar://21515402>

Swift SVN r30224