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Unlike many other platforms, Microsoft does not maintain ABI across debug and release binaries. When building LLVM in release+asserts, the injection of `_DEBUG` which controls the behaviour of `assert` as per the C specification, also alters the behaviour of the C/C++ runtime's internal assertions and relies on debug only symbols. As such, the `_DEBUG` macro handling is not _MSVC_ specific but rather MS STL and UCRT specific. Adjust the build condition from `MSVC` to `WIN32` to indicate that this is a Windows-ism. Note that `MINGW` is the proper way to check for MinGW, so that should not be impacted by this change. Making this more precise permits the use of `clang` and `clang++` to build LLVM for Windows.
See docs/CMake.html for instructions on how to build LLVM with CMake.