Removing the old, ad-hoc diagnostics code improves the diagnostics we
emit, since the existing diagnostics for missing conformances is already
pretty good.
rdar://127369509
This protocol appears in the stdlib as scaffolding for the
`NonescapableTypes` feature, which is still experimental and not gone
through evolution as an approved addition to the stdlib.
Rather than delete it from the stdlib, because it needs to still remain
to support that feature work, gate references to it behind a feature
flag.
Additionally, prevent documentation from seeing this declaration.
rdar://126705184
When a NoncopyableGenericsMismatch happens between the compiler and
stdlib, allow the compiler to rebuild the stdlib from its interface
instead of exiting with an error.
When the Swift module is not available, we'll synthesize the
Copyable/Escapable decls into the Builtin module.
In the future, it might be nice to just do this always, and define
typealiases for those types in the stdlib to refer to the ones in the
builtin module.
