[clang][dataflow] Print the source line if we saw unexpected diagnostics in tests.

[mboehme]

Summary

This makes it easier to determine which line the unexpected happened on; previously, we would only get the line number.

clang/unittests/Analysis/FlowSensitive/UncheckedOptionalAccessModelTest.cpp

 //===- UncheckedOptionalAccessModelTest.cpp -------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // FIXME: Move this to clang/unittests/Analysis/FlowSensitive/Models.
 
 #include "clang/Analysis/FlowSensitive/Models/UncheckedOptionalAccessModel.h"
 #include "TestingSupport.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Basic/SourceLocation.h"
+#include "clang/Frontend/TextDiagnostic.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Error.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <optional>
 #include <string>
llvm::Error Error = checkDataflow<UncheckedOptionalAccessModel>(
                               const AnalysisOutputs &AO) {
           llvm::DenseSet<unsigned> AnnotationLines;
           for (const auto &[Line, _] : Annotations) {
             AnnotationLines.insert(Line);
           }
           auto &SrcMgr = AO.ASTCtx.getSourceManager();
           llvm::DenseSet<unsigned> DiagnosticLines;
           for (SourceLocation &Loc : Diagnostics) {
-            DiagnosticLines.insert(SrcMgr.getPresumedLineNumber(Loc));
+            unsigned Line = SrcMgr.getPresumedLineNumber(Loc);
+            DiagnosticLines.insert(Line);
+            if (!AnnotationLines.contains(Line)) {
+              IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts(
+                  new DiagnosticOptions());
+              TextDiagnostic TD(llvm::errs(), AO.ASTCtx.getLangOpts(),
+                                DiagOpts.get());
+              TD.emitDiagnostic(
+                  FullSourceLoc(Loc, SrcMgr), DiagnosticsEngine::Error,
+                  "unexpected diagnostic", std::nullopt, std::nullopt);
+            }
           }
 
           EXPECT_THAT(DiagnosticLines, ContainerEq(AnnotationLines));
         });
     if (Error)
       FAIL() << llvm::toString(std::move(Error));
   }
 };