Disable availability checking when type-checking the _Concurrency interface

Swift 5.5 didn't support back deployment of concurrency features, so a
Swift 5.5 compiler processing the _Concurrency .swiftinterface will
produce errors for each `async` function and actor with pre-macOS
12/iOS 15 availability. Emit `-disable-availability-checking` into the
generated `.swiftinterface` files to allow Swift 5.5 to continue to
build them.

Finishes rdar://82602353.

Author: DougGregor

lib/Frontend/CompilerInvocation.cpp

@@ -370,6 +370,13 @@ static void SaveModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
     interleave(RenderedArgs,
                [&](const char *Argument) { PrintArg(OS, Argument, StringRef()); },
                [&] { OS << " "; });
+
+    // Backward-compatibility hack: disable availability checking in the
+    // _Concurrency module, so that older (Swift 5.5) compilers that did not
+    // support back deployment of concurrency do not complain about 'async'
+    // with older availability.
+    if (FOpts.ModuleName == "_Concurrency")
+      OS << " -disable-availability-checking";
   }
   {
     llvm::raw_string_ostream OS(Opts.IgnorableFlags);