[clang][tooling] Accept custom diagnostic options in ToolInvocation

This patch allows the clients of `ToolInvocation` to provide custom diagnostic options to be used during driver -> cc1 command-line transformation and parsing.

Tests covering this functionality are in a follow-up commit. To make this testable, the `DiagnosticsEngine` needs to be properly initialized via `CompilerInstance::createDiagnostics`.

Reviewed By: dexonsmith, arphaman

Differential Revision: https://reviews.llvm.org/D108976

clang/include/clang/Tooling/Tooling.h

@@ -268,11 +268,17 @@ public:
 
   ~ToolInvocation();
 
-  /// Set a \c DiagnosticConsumer to use during parsing.
+  /// Set a \c DiagnosticConsumer to use during driver command-line parsing and
+  /// the action invocation itself.
   void setDiagnosticConsumer(DiagnosticConsumer *DiagConsumer) {
     this->DiagConsumer = DiagConsumer;
   }
 
+  /// Set a \c DiagnosticOptions to use during driver command-line parsing.
+  void setDiagnosticOptions(DiagnosticOptions *DiagOpts) {
+    this->DiagOpts = DiagOpts;
+  }
+
   /// Run the clang invocation.
   ///
   /// \returns True if there were no errors during execution.
@@ -290,6 +296,7 @@ public:
   FileManager *Files;
   std::shared_ptr<PCHContainerOperations> PCHContainerOps;
   DiagnosticConsumer *DiagConsumer = nullptr;
+  DiagnosticOptions *DiagOpts = nullptr;
 };
 
 /// Utility to run a FrontendAction over a set of files.

clang/lib/Tooling/Tooling.cpp

@@ -343,10 +343,17 @@ bool ToolInvocation::run() {
   for (const std::string &Str : CommandLine)
     Argv.push_back(Str.c_str());
   const char *const BinaryName = Argv[0];
-  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts =
-      CreateAndPopulateDiagOpts(Argv);
-  TextDiagnosticPrinter DiagnosticPrinter(
-      llvm::errs(), &*DiagOpts);
+
+  // Parse diagnostic options from the driver command-line only if none were
+  // explicitly set.
+  IntrusiveRefCntPtr<DiagnosticOptions> ParsedDiagOpts;
+  DiagnosticOptions *DiagOpts = this->DiagOpts;
+  if (!DiagOpts) {
+    ParsedDiagOpts = CreateAndPopulateDiagOpts(Argv);
+    DiagOpts = &*ParsedDiagOpts;
+  }
+
+  TextDiagnosticPrinter DiagnosticPrinter(llvm::errs(), DiagOpts);
   DiagnosticsEngine Diagnostics(
       IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs()), &*DiagOpts,
       DiagConsumer ? DiagConsumer : &DiagnosticPrinter, false);