[CSApply] Allow marker existential to superclass conversions
If existential is a protocol composition type where all of the
protocols are `@_marker`, narrowly allow coercion to its superclass
bound (if it matches). Both types have the same representation
which makes it okay.
This is only a problem in Swift 5 mode without strict concurrency
checks. In this mode `@preconcurrency` stripping happens
outside of the solver which means that no conversion restrictions
are recorded for members that got `& Sendable` stripped from
their types.
For example:
```swift
struct S {
@preconcurrency static let member: KeyPath<String, Int> & Sendable
}
func test() {
_ = S.member
}
```
Since `member` is `@preconcurrency` its type would get concurrency
annotations stripped, which includes `& Sendable` which means that
the solver uses `KeyPath<String, Int>` type for the reference and
not the original `KeyPath<String, Int> & Sendable`, this is a problem
for `ExprRewritter::adjustTypeForDeclReference` because conversion
between existential and its superclass bound requires a constraint
restriction which won't be available in this case.
Resolves: rdar://132700409
(cherry picked from commit 38aa71de91e31c9d0ed16d93db76c09b25c1ae8d)
