swift234

[noescape by default] drop isExplicitlyEscaping bit

The isExplicitlyEscaping bit, though useful for printing,
unfortunately puts us in a position where we have different bit
patterns for the same type, and thus lose much of our type equivalence
checking for overriding, protocol conformance, etc., even if we were
to take subtyping into account. We need to drop it, relying on the
existing noescape bit alone to determine the type's semantics (at
least, as long as we continue to encode this information in the type
system).

This is a partial fix; we will now be excessively printing @escaping,
but the subsequent commits will correct this. For printing, we will
instead need to be more context-aware.