Make diagnostic verification work after the redesign of #488.

Re-enable diagnostic verification in regression tests where it still
succeeds. It remains disabled in some tests in which failures were
introduced while it was disabled; #609 is for that.

Fixes #503.

clang/include/clang/3C/3C.h

@@ -99,6 +99,8 @@ class _3CInterface {
          const std::vector<std::string> &SourceFileList,
          clang::tooling::CompilationDatabase *CompDB);
 
+  ~_3CInterface();
+
   // Call clang to provide the data
   bool parseASTs();
 
@@ -136,10 +138,25 @@ class _3CInterface {
   // Dump all stats related to performance.
   bool dumpStats();
 
+  // Determine the exit code that the `3c` tool should exit with after the last
+  // 3C stage, considering diagnostic verification. Must be called exactly once
+  // before the _3CInterface is destructed (unless construction failed).
+  int determineExitCode();
+
 private:
   _3CInterface(const struct _3COptions &CCopt,
                const std::vector<std::string> &SourceFileList,
-               clang::tooling::CompilationDatabase *CompDB, bool &Failed);
+               clang::tooling::CompilationDatabase *CompDB);
+
+  bool ConstructionFailed = false;
+  bool DeterminedExitCode = false;
+
+  bool HadNonDiagnosticError = false;
+
+  // Determine whether 3C can continue to the next stage of processing. Checks
+  // HadNonDiagnosticError and error diagnostics but ignores diagnostic
+  // verification.
+  bool isSuccessfulSoFar();
 
   // saved ASTs
   std::vector< std::unique_ptr< ASTUnit >> ASTs;