swift234

fix <rdar://problem/19782264> Immutable, optional class members can't have their subproperties read from during init()

The problem here was that the _preconditionImplicitlyUnwrappedOptionalHasValue
compiler intrinsic was taking the optional/IUO argument as inout as a performance
optimization, but DI would reject it (in narrow cases, in inits) because the inout
argument looks like a mutation.  

We could rework this to take it as an @in argument or something, but it is better
to just define this problem away: the precondition doesn't actually care about the
optional, it is just testing its presence, which SILGen does all the time.  Have
SILGen open code the switch_enum and just have the stdlib provide a simpler
_diagnoseUnexpectedNilOptional() to produce the error message.

This avoids the problem completely and produces slightly better -O0 codegen.



Swift SVN r25254