Rename SILBuilder::createUncheckedBitCast
to SILBuilder::createUncheckedForwardingCast
It would be disastrous to confuse this utility with a bit cast. A bit
cast always produces an Unowned value which must immediately be copied
to be used. This utility always forwards ownership. It cannot be used
to truncate values.
Also, be careful not to convert "reinterpret cast"
(e.g. Builtin.reinterpretCast) into a "value cast" since ownership
will be incorrect and the reinterpreted types might not have
equivalent layout.
