Use raw module format to fix -gmodules crash

To avoid `llvm::report_fatal_error("unknown module format")` in getPCHContainerWriter.
See llvm/llvm-project@a3e2ff1

src/clang_tu.cc

@@ -175,6 +175,8 @@ buildCompilerInvocation(const std::string &main, std::vector<const char *> args,
   ci->getPreprocessorOpts().ImplicitPCHInclude.clear();
   ci->getPreprocessorOpts().PrecompiledPreambleBytes = {0, false};
   ci->getPreprocessorOpts().PCHThroughHeader.clear();
+
+  ci->getHeaderSearchOpts().ModuleFormat = "raw";
   return ci;
 }