[clang][Driver] Don't overwrite `DiagnosticsEngine::IgnoreAllWarnings`, rely on `DiagnosticOptions::IgnoreWarnings` value.

[vsapsai]

Summary

Driver overwrites DiagnosticsEngine::IgnoreAllWarnings based on -w flag
without taking into account DiagnosticOptions::IgnoreWarnings that is
propagated to DiagnosticsEngine in ProcessWarningOptions (called from
CompilerInstance::createDiagnostics). It makes it hard to manipulate
DiagnosticOptions directly and pushes towards string-based API.

Most of in-tree tools use DiagnosticOptions already, so migrate
clang_parseTranslationUnit_Impl to use it too. Don't parse -w
directly but rely on

def w : Flag<["-"], "w">, HelpText<"Suppress all warnings">, Flags<[CC1Option]>,
  MarshallingInfoFlag<DiagnosticOpts<"IgnoreWarnings">>;

Allows to reland D138252.

[vsapsai]

To help folks digging up the history, the code in driver was introduced in https://reviews.llvm.org/D11322 and the test case added in "test/Index/warning-flags.c" requires the change in "clang/tools/libclang/CIndex.cpp", that's why I don't have any extra test changes.

[vsapsai]

Thanks for the review!

clang/lib/Driver/Driver.cpp

Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
   // Check for working directory option before accessing any files
   if (Arg *WD = Args.getLastArg(options::OPT_working_directory))
     if (VFS->setCurrentWorkingDirectory(WD->getValue()))
       Diag(diag::err_drv_unable_to_set_working_directory) << WD->getValue();
 
   // FIXME: This stuff needs to go into the Compilation, not the driver.
   bool CCCPrintPhases;
 
-  // Silence driver warnings if requested
-  Diags.setIgnoreAllWarnings(Args.hasArg(options::OPT_w));
-
   // -canonical-prefixes, -no-canonical-prefixes are used very early in main.
   Args.ClaimAllArgs(options::OPT_canonical_prefixes);
   Args.ClaimAllArgs(options::OPT_no_canonical_prefixes);
 
   // f(no-)integated-cc1 is also used very early in main.
   Args.ClaimAllArgs(options::OPT_fintegrated_cc1);
   Args.ClaimAllArgs(options::OPT_fno_integrated_cc1);
 

clang/tools/libclang/CIndex.cpp

clang_parseTranslationUnit_Impl(CXIndex CIdx, const char *source_filename,
   if (options & CXTranslationUnit_SkipFunctionBodies) {
     SkipFunctionBodies =
         (options & CXTranslationUnit_LimitSkipFunctionBodiesToPreamble)
             ? SkipFunctionBodiesScope::Preamble
             : SkipFunctionBodiesScope::PreambleAndMainFile;
   }
 
   // Configure the diagnostics.
+  std::unique_ptr<DiagnosticOptions> DiagOpts = CreateAndPopulateDiagOpts(
+      llvm::makeArrayRef(command_line_args, num_command_line_args));
   IntrusiveRefCntPtr<DiagnosticsEngine> Diags(
-      CompilerInstance::createDiagnostics(new DiagnosticOptions));
+      CompilerInstance::createDiagnostics(DiagOpts.release()));
 
   if (options & CXTranslationUnit_KeepGoing)
     Diags->setFatalsAsError(true);
 
   CaptureDiagsKind CaptureDiagnostics = CaptureDiagsKind::All;
   if (options & CXTranslationUnit_IgnoreNonErrorsFromIncludedFiles)
     CaptureDiagnostics = CaptureDiagsKind::AllWithoutNonErrorsFromIncludes;