swift234

SILGen: Introduce MaterializedLValue abstraction

This is used when materializing an LValue to share state between
the read and write phases of the access, replacing the 'temporary'
and 'extraInfo' parameters that were previously being passed around.

It adds two new fields, origSelfType and genericSig, which will be
used in an upcoming patch to actually apply the callback with the
current generic signature.

This will finally allow us to make use of materializeForSet
implementations in protocol extensions, which is a prerequisite
for enabling resilient default implementations of property and
subscript requirements.