Sema: Relax availability checking in universally unavailable contexts.
Recent refactoring fixed a bug that previously caused `f()` to be checked as if
it were unavailable only on macOS in the following example:
```
@available(macOS, unavailable)
struct Outer {
@available(*, unavailable)
func f() {
someFunctionUnavailableOnMacOS()
}
}
```
Unfortunately, fixing that bug made a different existing availability checking
rule more problematic. References to declarations that are unavailable on the
current platform have been diagnosed as unavailable even in contexts that are
universally unavailable. This long standing behavior is overly strict but it
rarely had consequences. However, now that the example above is modeled
correctly, this overly strict behavior is causing some source compatibility
issues. The easiest solution is to relax the overly strict checking.
Resolves rdar://141124478.
swift
eafea5b2 - Sema: Relax availability checking in universally unavailable contexts.
