[Diagnostics] Make sure extension type repr is available before using it

Before suggesting to add `where Self == ...` condition to the extension
of a protocol involved in a static member lookup, let's check whether
type repr is available and valid.

Resolves: rdar://78097387
(cherry picked from commit a7988b0)

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -7433,9 +7433,15 @@ bool InvalidMemberRefOnProtocolMetatype::diagnoseAsError() {
   if (auto *whereClause = extension->getTrailingWhereClause()) {
     auto sourceRange = whereClause->getSourceRange();
     note.fixItInsertAfter(sourceRange.End, ", Self == <#Type#> ");
-  } else {
-    auto nameRepr = extension->getExtendedTypeRepr();
-    note.fixItInsertAfter(nameRepr->getEndLoc(), " where Self == <#Type#>");
+  } else if (auto nameRepr = extension->getExtendedTypeRepr()) {
+    // Type repr is not always available so we need to be defensive
+    // about its presence and validity.
+    if (nameRepr->isInvalid())
+      return true;
+
+    if (auto noteLoc = nameRepr->getEndLoc()) {
+      note.fixItInsertAfter(noteLoc, " where Self == <#Type#>");
+    }
   }
 
   return true;