[ClangImporter] Undo changes around createClangInvocation (followup to #85445) (#85457)

Feedback in #85445 after it merged pointed out that the changes around
`createClangInvocation` are not necessary because `CompilerInvocation`
do not hold a reference to `clang::DiagnosticOptions`, only the
`clang::driver::Driver` does.

These changes undo the modifications done there and return the code to
the previous state (but keeps the changes around `createClangDriver`
which was causing the use-after-free).

Author: drodriguez

include/swift/ClangImporter/ClangImporter.h

@@ -216,8 +216,7 @@ class ClangImporter final : public ClangModuleLoader {
   std::vector<std::string>
   getClangDepScanningInvocationArguments(ASTContext &ctx);
 
-  static std::pair<std::unique_ptr<clang::CompilerInvocation>,
-                   std::unique_ptr<clang::DiagnosticOptions>>
+  static std::unique_ptr<clang::CompilerInvocation>
   createClangInvocation(ClangImporter *importer,
                         const ClangImporterOptions &importerOpts,
                         llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,

lib/ClangImporter/ClangImporter.cpp

@@ -1284,9 +1284,7 @@ std::optional<std::vector<std::string>> ClangImporter::getClangCC1Arguments(
   return CI->getCC1CommandLine();
 }
 
-std::pair<std::unique_ptr<clang::CompilerInvocation>,
-          std::unique_ptr<clang::DiagnosticOptions>>
-ClangImporter::createClangInvocation(
+std::unique_ptr<clang::CompilerInvocation> ClangImporter::createClangInvocation(
     ClangImporter *importer, const ClangImporterOptions &importerOpts,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,
     const std::vector<std::string> &CC1Args) {
@@ -1300,19 +1298,19 @@ ClangImporter::createClangInvocation(
   // option here is either generated by dependency scanner or just round tripped
   // from `getClangCC1Arguments` so we don't expect it to fail. Use a simple
   // printing diagnostics consumer for debugging any unexpected error.
-  auto diagOpts = std::make_unique<clang::DiagnosticOptions>();
+  clang::DiagnosticOptions diagOpts;
   clang::DiagnosticsEngine clangDiags(
-      new clang::DiagnosticIDs(), *diagOpts,
-      new clang::TextDiagnosticPrinter(llvm::errs(), *diagOpts));
+      new clang::DiagnosticIDs(), diagOpts,
+      new clang::TextDiagnosticPrinter(llvm::errs(), diagOpts));
 
   // Finally, use the CC1 command-line and the diagnostic engine
   // to instantiate our Invocation.
   auto CI = std::make_unique<clang::CompilerInvocation>();
   if (!clang::CompilerInvocation::CreateFromArgs(
           *CI, invocationArgs, clangDiags, importerOpts.clangPath.c_str()))
-    return {nullptr, nullptr};
+    return nullptr;
 
-  return {std::move(CI), std::move(diagOpts)};
+  return CI;
 }
 
 std::unique_ptr<ClangImporter> ClangImporter::create(
@@ -1360,9 +1358,8 @@ std::unique_ptr<ClangImporter> ClangImporter::create(
                        [] { llvm::errs() << "' '"; });
       llvm::errs() << "'\n";
     }
-    std::tie(importer->Impl.Invocation, importer->Impl.DiagnosticOptions) =
-        createClangInvocation(importer.get(), importerOpts, VFS,
-                              importer->Impl.ClangArgs);
+    importer->Impl.Invocation = createClangInvocation(
+        importer.get(), importerOpts, VFS, importer->Impl.ClangArgs);
     if (!importer->Impl.Invocation)
       return nullptr;
   }
@@ -1438,7 +1435,7 @@ std::unique_ptr<ClangImporter> ClangImporter::create(
         ctx, instance.getVirtualFileSystemPtr(), true);
     if (!swiftTargetClangArgs)
       return nullptr;
-    auto [swiftTargetClangInvocation, clangDiagOpts] = createClangInvocation(
+    auto swiftTargetClangInvocation = createClangInvocation(
         importer.get(), importerOpts, instance.getVirtualFileSystemPtr(),
         *swiftTargetClangArgs);
     if (!swiftTargetClangInvocation)

lib/ClangImporter/ImporterImpl.h

@@ -551,9 +551,6 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   /// Clang arguments used to create the Clang invocation.
   std::vector<std::string> ClangArgs;
 
-  /// Clang diagnostic options used to create the Clang invocation.
-  std::unique_ptr<clang::DiagnosticOptions> DiagnosticOptions;
-
   /// Mapping from Clang swift_attr attribute text to the Swift source file(s)
   /// that contain that attribute text.
   ///