Linker directives within the .drectve section of generated COFF object files are unnecessarily quoted, slowing down MSVC LINK.EXE significantly

[januszn]

This was observed with vanilla Clang 16.0.6 built from source, and 16.0.5 distributed by MS with Visual Studio 2022 17.7.4, however the offending code is in the lower-level LLVM code, not Clang's. In

llvm-project/llvm/lib/IR/Mangler.cpp

Line 194 in 7cbf1a2

return isAlnum(C) || C == '_' || C == '@';

, which determines which characters in a symbol's name require it to be quoted within a linker directive, the $ and ? characters, both common in the MS ABI mangling scheme, are not exempted. This results in Clang emitting /EXPORT directives with the names quoted for most C++ symbols to be exported from a DLL. The MSVC linker has a massive performance issue with parsing .drectve sections when either linking a DLL or creating an import library from the Clang-built .OBJ files that becomes apparent when there are enough exported symbols. And since llvm-lib doesn't support the /DEF switch, using LIB.EXE from the MSVC toolchain (which just calls LINK.EXE /LIB) is required if the DLLs to be built have circular dependencies (see https://learn.microsoft.com/en-us/cpp/build/reference/using-an-import-library-and-export-file?view=msvc-170), which is the case for Unreal Editor.

Detailed repro steps tailored for the corresponding issue in the MSVC linker are at: https://developercommunity.visualstudio.com/t/Poor-performance-linking-DLLs-with-LINK/10483519

The LLVM-side fix appears simple - don't trigger quoting names with '$' and '?' if (TT.isWindowsMSVCEnvironment()) and I can quickly create a PR with that fix - are there perhaps issues with that approach that I'm not seeing?