Add DiagGroupID to Diagnostic

This change addresses the following issue: when an error is being wrapped in a warning, the diagnostic message will use the wrapper's `DiagGroupID` as the warning's name. However, we want to retain the original error's group for use. For example, in Swift 5, async_unavailable_decl is wrapped in error_in_future_swift_version. When we print a diagnostic of this kind, we want to keep the `DiagGroupID` of `async_unavailable_decl`, not that of `error_in_future_swift_version`.
To achieve this, we add `DiagGroupID` to the `Diagnostic` class. When an active diagnostic is wrapped in DiagnosticEngine, we retain the original `DiagGroupID`.

For illustration purposes, this change also introduces a new group: `DeclarationUnavailableFromAsynchronousContext`.

With this change, we produce errors and warnings of this kind with messages like the following:

```
global function 'fNoAsync' is unavailable from asynchronous contexts [DeclarationUnavailableFromAsynchronousContext]
global function 'fNoAsync' is unavailable from asynchronous contexts; this is an error in the Swift 6 language mode [DeclarationUnavailableFromAsynchronousContext]
```

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -977,8 +977,8 @@ public:
                       const_cast<ValueDecl *>(vd));
         // Emit a specific unavailable message when we know why a decl can't be
         // exposed, or a generic message otherwise.
-        auto diagString = M.getASTContext().Diags.diagnosticStringFor(
-            diag.getID(), PrintDiagnosticNamesMode::None);
+        auto diagString =
+            M.getASTContext().Diags.diagnosticStringFor(diag.getID());
         DiagnosticEngine::formatDiagnosticText(os, diagString, diag.getArgs(),
                                                DiagnosticFormatOptions());
         os << "\");\n";