Issue Description:

Currently, fixAvailabilityForDecl() in TypeCheckAvailability.cpp exits early if there is already an @available attribute on the declaration, with a comment noting work that should be done in the future:

/// Emit a diagnostic note and Fix-It to add an @available attribute
/// on the given declaration for the given version range.
static void fixAvailabilityForDecl(SourceRange ReferenceRange, const Decl *D,
                                   const VersionRange &RequiredRange,
                                   ASTContext &Context) {
  // ...irrelevant code omitted...

  if (getActiveAvailableAttribute(D, Context)) {
    // For QoI, in future should emit a fixit to update the existing attribute.
    return;
  }

Indeed, we should make this change. For instance, if you give the compiler this code:

@available(macOS 42, *) func foo() {}

@available(macOS 12, *) func bar() {
    foo()
}

Swift should emit a note with a fix-it replacing "macOS 12" with "macOS 42".

Your implementation, should you choose to commit it, should test not only this easy case with one simple @available attribute, but also cases with multiple OSes and long-form @available attributes. You will certainly need to update some existing tests; those might adequately cover this, but I'm not sure.