[Windows] LLVM build failure with VS Clang 20.1

[psiha]

Trying to

• build LLVM (tried master and the 21.1.7 tag)
• under Windows
• using the LLVM 20 toolset shipped with Visual Studio 2026
• using MSVC or ClangCL works but trying with Clang 'proper' (the standard GNU interface) the build fails with this linker error:
  lld-link: error: undefined symbol: void * __cdecl operator new[](unsigned __int64, class clang::ASTContext const &, unsigned __int64))
  (using LTO in all cases)