[Diagnostics] Modify diagnostics to suggest updating existing available attribute

[mininny]

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".

Resolves SR-15051

[mininny]

I'm not sure why but adding expected fixit note like:

@available(OSX 10.0, *) // expected-note {{16-20=10.99}}

didn't work. (maybe because the original note is from the struct EntryPointline?)
So I put the two expressions in the same line, but is there a proper way to fix this?

[mininny]

Wondering if you had a chance to look at this :) @beccadax