[Diags] Avoid infinite recursion on decl printing request cycle

The ASTPrinter may kick requests that may emit diagnostics, make sure
we don't attempt to recursively print the decl since that would lead
to infinite recursion when diagnosing the cycle.

Author: hamishknight

lib/AST/DiagnosticEngine.cpp

@@ -1410,11 +1410,15 @@ DiagnosticEngine::diagnosticInfoForDiagnostic(const Diagnostic &diagnostic,
     loc = decl->getLoc();
 
     // If the location of the decl is invalid still, try to pretty-print the
-    // declaration into a buffer and capture the source location there.
+    // declaration into a buffer and capture the source location there. Make
+    // sure we don't have an active request running since printing AST can
+    // kick requests that may themselves emit diagnostics. This won't help the
+    // underlying cycle, but it at least stops us from overflowing the stack.
     if (loc.isInvalid()) {
-      loc = evaluateOrDefault(
-          decl->getASTContext().evaluator, PrettyPrintDeclRequest{decl},
-          SourceLoc());
+      PrettyPrintDeclRequest req(decl);
+      auto &eval = decl->getASTContext().evaluator;
+      if (!eval.hasActiveRequest(req))
+        loc = evaluateOrDefault(eval, req, SourceLoc());
     }
   }