swift234

Introduce thick/Objective-C metatype conversion instructions.

Introduce the SIL instructions thick_to_objc_metatype and
objc_to_thick_metatype to convert between the 'thick' and
'Objective-C' representations of a metatype. Most of this code is
trivial support code for these conversions: printing, parsing,
(de-)serialization, etc., for which testing will come online in
subsequent patches or is incidental in other tests.

Lower Objective-C metatype values down to objc_class* at the IR level
and implement IRGen support for these SIL instructions. SIL-only test
case at the moment because SILGen never creates these instructions.



Swift SVN r14087