Allow any declaration to be marked with `@unsafe`, meaning that it
involves unsafe code. This also extends to C declarations marked with
the `swift_attr("unsafe")` attribute.
Under a separate experimental flag (`DisallowUnsafe`), diagnose any
attempt to use an `@unsafe` declaration or any unsafe language feature
(such as `unowned(unsafe)`, `@unchecked Sendable`). This begins to
define a "safe" mode in Swift that prohibits memory-unsafe constructs.
The attribute declares that a struct contains "sensitive" data.
It enforces that the contents of such a struct value is zeroed out at the end of its lifetime.
In other words: the content of such a value is not observable in memory after the value's lifetime.
Also add an experimental feature `Sensitive` with which the attribute can be enabled.
Our standard conception of suppressible features assumes we should
always suppress the feature if the compiler doesn't support it.
This presumes that there's no harm in suppressing the feature, and
that's a fine assumption for features that are just adding information
or suppressing new diagnostics. Features that are semantically
relevant, maybe even ABI-breaking, are not a good fit for this,
and so instead of reprinting the decl with the feature suppressed,
we just have to hide the decl entirely. The missing middle here
is that it's sometimes useful to be able to adopt a type change
to an existing declaration, and we'd like older compilers to be
able to use the older version of the declaration. Making a type
change this way is, of course, only really acceptable for
@_alwaysEmitIntoClient declarations; but those represent quite a
few declarations that we'd like to be able to refine the types of.
Rather than trying to come up with heuristics based on
@_alwaysEmitIntoClient or other sources of information, this design
just requires the declaration to opt in with a new attribute,
@_allowFeatureSuppress. When a declaration opts in to suppression
for a conditionally-suppressible feature, the printer uses the
suppression serially-print-with-downgraded-options approach;
otherwise it uses the print-only-if-feature-is-available approach.
Instead of using dummy `_counting_KAK_*` global symbols, use
`LAST_DECL_ATTR` metaprogramming technique to determine the number of
enum values. This align with other "kind" enum e.g. `DeclKind`.
