Set PreprocessorOpts.GeneratePreamble=true in PrecompiledPreamble.

[ilya-biryukov]

Summary

It was previsouly set only in ASTUnit, but it should be set for all client of
PrecompiledPreamble.

[ilya-biryukov]

This commit will unbreak clangd from all the same issues that were fixed by conditional stack commits.

[ilya-biryukov]

Adding more reviewers, in case someone will have time to look at that :-)

[bkramer]

A testcase would be nice, but this can go in to unblock things.

[ilya-biryukov]

In D38617#892092, @bkramer wrote:

A testcase would be nice, but this can go in to unblock things.

Thanks for reviewing this!
Added a test case to clangd in rL315213.

lib/Frontend/ASTUnit.cpp

ASTUnit *ASTUnit::LoadFromCommandLine(
   // Override any files that need remapping
   for (const auto &RemappedFile : RemappedFiles) {
     CI->getPreprocessorOpts().addRemappedFile(RemappedFile.first,
                                               RemappedFile.second);
   }
   PreprocessorOptions &PPOpts = CI->getPreprocessorOpts();
   PPOpts.RemappedFilesKeepOriginalName = RemappedFilesKeepOriginalName;
   PPOpts.AllowPCHWithCompilerErrors = AllowPCHWithCompilerErrors;
-  PPOpts.GeneratePreamble = PrecompilePreambleAfterNParses != 0;
   PPOpts.SingleFileParseMode = SingleFileParse;
   
   // Override the resources path.
   CI->getHeaderSearchOpts().ResourceDir = ResourceFilesPath;
 
   CI->getFrontendOpts().SkipFunctionBodies = SkipFunctionBodies;
 
   if (ModuleFormat)

lib/Frontend/PrecompiledPreamble.cpp

llvm::ErrorOr<PrecompiledPreamble> PrecompiledPreamble::Build(
   bool PreambleEndsAtStartOfLine = Bounds.PreambleEndsAtStartOfLine;
 
   // Tell the compiler invocation to generate a temporary precompiled header.
   FrontendOpts.ProgramAction = frontend::GeneratePCH;
   // FIXME: Generate the precompiled header into memory?
   FrontendOpts.OutputFile = PreamblePCHFile->getFilePath();
   PreprocessorOpts.PrecompiledPreambleBytes.first = 0;
   PreprocessorOpts.PrecompiledPreambleBytes.second = false;
+  // Inform preprocessor to record conditional stack when building the preamble.
+  PreprocessorOpts.GeneratePreamble = true;
 
   // Create the compiler instance to use for building the precompiled preamble.
   std::unique_ptr<CompilerInstance> Clang(
       new CompilerInstance(std::move(PCHContainerOps)));
 
   // Recover resources if we crash before exiting this method.
   llvm::CrashRecoveryContextCleanupRegistrar<CompilerInstance> CICleanup(
       Clang.get());