swift234

Generate method/property @encodings from the foreign
SILFunctionType of the method instead of its formal type.

Gives more accurate information to the @encoding, makes
foreign error conventions work implicitly, and allows
IRGen's Swift-to-Clang to avoid duplicating arbitrary
amounts of the bridging logic from SILGen.

Some finagling was required in order to avoid calling
getConstantFunctionType from within other kinds of
lowering, which might have re-entered a generic context.

Also required fixing a bug with the type lowering of
optional DynamicSelfTypes where we would end up with
a substituted type in the lowered type.

Also, for some reason, our @encoding for -dealloc
methods was pretending that there was a formal parameter.
There didn't seem to be any justification for this,
and it's not like Clang does that.  Fixed.

This commit reapplies r29266 with a conservative build fix
that disables ObjC property descriptors for @objc properties
that lack a getter.  That should only be possible in SIL
files, because @objc should force accessors to be synthesized.
Arguably, Sema shouldn't be marking things implicitly @objc
in SIL files, but I'll leave that decision open for now.

Swift SVN r29272