[clangd] Always default to raw pch format

[kadircet]

Summary

Clang would emit a fatal error when it encounters an unregistered PCH
format. This change ensures clangd will always use raw format no matter what
user specifies.

As side effects:

• serializing an AST in an unknown format might throw off build

systems. I suppose this would only be an issue when build system and clangd are
racing for same PCM modules, hopefully this should be rare and both clangd or
the build system should recover on the next run.

• whenever clang reads a serialized AST it seems to be checking for file

signature and emitting non-fatal errors. so this should be fine again.

The only other valid module format in clang is obj but it is part of codegen,
i don't think it is worth the dependency. Hence chosing to not register it, at
least yet.

clang-tools-extra/clangd/Compiler.cpp

buildCompilerInvocation(const ParseInputs &Inputs, clang::DiagnosticConsumer &D,
   CI->getPreprocessorOpts().ImplicitPCHInclude.clear();
   CI->getPreprocessorOpts().PrecompiledPreambleBytes = {0, false};
   CI->getPreprocessorOpts().PCHThroughHeader.clear();
   CI->getPreprocessorOpts().PCHWithHdrStop = false;
   CI->getPreprocessorOpts().PCHWithHdrStopCreate = false;
   // Don't crash on `#pragma clang __debug parser_crash`
   CI->getPreprocessorOpts().DisablePragmaDebugCrash = true;
 
+  // Always default to raw container format as clangd doesn't registry any other
+  // and clang dies when faced with unknown formats.
+  CI->getHeaderSearchOpts().ModuleFormat =
+      PCHContainerOperations().getRawReader().getFormat().str();
+
   return CI;
 }
 
 std::unique_ptr<CompilerInstance>
 prepareCompilerInstance(std::unique_ptr<clang::CompilerInvocation> CI,
                         const PrecompiledPreamble *Preamble,
                         std::unique_ptr<llvm::MemoryBuffer> Buffer,
                         llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,

clang-tools-extra/clangd/unittests/ModulesTests.cpp

module M {
       header "modular.h"
     }
 )modulemap";
 
   // Test that we do not crash.
   TU.build();
 }
 
+// Unknown module formats are a fatal failure for clang. Ensure we don't crash.
+TEST(Modules, UnknownFormat) {
+  TestTU TU = TestTU::withCode(R"(#include "modular.h")");
+  TU.OverlayRealFileSystemForModules = true;
+  TU.ExtraArgs.push_back("-Xclang");
+  TU.ExtraArgs.push_back("-fmodule-format=obj");
+  TU.ExtraArgs.push_back("-fmodule-map-file=" + testPath("m.modulemap"));
+  TU.ExtraArgs.push_back("-fmodules");
+  TU.ExtraArgs.push_back("-fimplicit-modules");
+  TU.AdditionalFiles["modular.h"] = "";
+  TU.AdditionalFiles["m.modulemap"] = R"modulemap(
+    module M {
+      header "modular.h"
+    })modulemap";
+
+  // Test that we do not crash.
+  TU.build();
+}
 } // namespace
 } // namespace clangd
 } // namespace clang