swift234

AST: Tighten up AbstractStorageDecl::isSettable()

'let' properties are settable inside certain initializers, but the
logic was wrong -- in most cases we would fall through and return
true from ::isSettable().

This came up when deserialization (correctly) didn't set the setter
accessibility on a 'let' property, and a read of that property from
a constructor in another struct incorrectly decided the property
was settable, causing a crash.

Fixes <rdar://problem/21559246>.

Swift SVN r30484