Build failure

[mattgodbolt]

Hey there! I don't want to pick at old wounds, but the build we use of llvm-embed on Compiler Explorer has been failing for a while now, and I figured I'd report it here. https://github.com/compiler-explorer/infra/runs/4069281512?check_suite_focus=true (open the "Daily build" and then "docker build" (I'm still working on this...)

The errors appear to be:

2021-11-01T15:27:20.8683239Z In file included from /root/llvm-project/clang/include/clang/Lex/PreprocessingRecord.h:20:0,
2021-11-01T15:27:20.8685411Z                  from /root/llvm-project/clang/include/clang/Frontend/ASTUnit.h:27,
2021-11-01T15:27:20.8716795Z                  from /root/llvm-project/clang/include/clang/Frontend/FrontendAction.h:23,
2021-11-01T15:27:20.8718834Z                  from /root/llvm-project/clang/include/clang/ARCMigrate/ARCMTActions.h:13,
2021-11-01T15:27:20.8721150Z                  from /root/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:14:
2021-11-01T15:27:20.8725461Z /root/llvm-project/clang/include/clang/Lex/PPCallbacks.h:603:8: error: 'void clang::PPChainedCallbacks::HasEmbed(clang::SourceLocation, llvm::StringRef, bool, llvm::Optional<clang::FileEntryRef>)' marked 'override', but does not override
2021-11-01T15:27:20.8953022Z    void HasEmbed(SourceLocation Loc, StringRef FileName, bool IsAngled,
2021-11-01T15:27:20.8954273Z         ^~~~~~~~
2021-11-01T15:27:20.8958557Z /root/llvm-project/clang/include/clang/Lex/PPCallbacks.h:379:16: warning: 'virtual void clang::PPCallbacks::HasEmbed(clang::SourceLocation, llvm::StringRef, bool, llvm::Optional<clang::FileEntryRef>, clang::SrcMgr::CharacteristicKind)' was hidden [-Woverloaded-virtual]

which might be a fault in our build process (see compiler-explorer/clang-builder#28 for details). I'm going to disable the build for now, but if there's a quick fix, we're all ears :)

Thanks as ever!