Sema: Use the availability of the extended nominal as a floor for the availability of extensions

[tshortli]

Many API libraries contain public extension decls that don't specify availability because they introduce additional members to the extended type in the same releases that the extended type was declared. Others contain extensions where the extension itself does not have declared availability but each of the members do. In both cases, the code works as written because extension members cannot be used without referencing the extended type.

@available(macOS 11, *)
struct S { }

// No availability, but every member of this extension has existed since `S` was introduced.
extension S {
  public func foo() { }
}

// No availability on the extension, but every public member has its own availability annotation. 
extension S {
  @available(macOS 12, *)
  public func bar() { }
}

With -target-min-inlining-version min specified, these extensions are diagnosed because the default availability floor for public decls is lowered below the deployment target.

The changes in this PR make the compiler use the availability of the extended nominal as a floor when checking availability in an extension decl to avoid unnecessary diagnostics for API libraries.

Resolves rdar://93630782

[tshortli]

@swift-ci please test