Report unnecessary availability warnings independently from the deployment target (#37394)
This change separates emission of the diagnostics like:
```
unnecessary check for 'Platform'; enclosing scope ensures guard will always be true
```
from the deployment target of the current compilation. Instead, these diagnostics will only be emitted if the enclosing scope guard is explicitly specified by the user with an `#availability` attribute.
This fixes cases like the following:
```
@available(macOS 11.0, *)
class Foo {
func foo() {
if #available(macOS 11.1, *) {}
}
}
```
Compiling this with `-target x86_64-apple-macos11.2` results in:
```
warning: unnecessary check for 'macOS'; enclosing scope ensures guard will always be true
if #available(macOS 11.1, *) {}
.../test.swift:2:7: note: enclosing scope here
class Foo {
```
Even though in source-code the enclosing scope (`Foo`) of this guard does not ensure it will always be true.
This happens because availability range is propagated by intersecting ranges top-down from the root `TypeRefinementContext`, which is defined by the deployment target, causing the availability range on class `Foo` to become `11.2`.
Users may be sharing the same piece of source-code across different projects with their own respective deployment targets which makes such target-dependent warnings confusing at some times, and not-useful at others.
We should rather have the warning simply reflect what is in the source.
Resolves rdar://77607488
swift
4c2b5646 - Report unnecessary availability warnings independently from the deployment target (#37394)
