swift234

Do even less availability checking under -disable-availability-checking. (#4214)

...and make sure we're in that mode for SIL inputs and for sil-opt and
sil-extract, even when working with AST types and declarations rather
than SIL types.

Without this, we get zillions of deprecation warnings coming out of
the validation tests SIL/parse_stdlib_*.sil, which dump the standard
library and then attempt to re-parse it. This has been causing the
"long" tests to take, well, too long.