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.
Availability checking for types was only suppressed when the immediate context
for the use of the type was explicitly marked unavailable. Availability is
lexical so the checking should be suppressed in the entire scope instead.
10.50 was once greater than any real macOS version, but now it compares
less than real released versions, which makes these tests depend on the
deployment target unnecessarily. Update these tests to use even larger
numbers to hopefully keep them independent a little longer.
