Note the following excerpts from MSYS2 documentation:
Name Prefix Toolchain Architecture C Library C++ Library
MINGW64 /mingw64 gcc x86_64 msvcrt libstdc++
UCRT64 /ucrt64 gcc x86_64 ucrt libstdc++
CLANG64 /clang64 llvm x86_64 ucrt libc++
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GCC vs LLVM/Clang:
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GCC based environments:
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While there also exists a Clang package in the MINGW environments, that one still uses the GNU linker and the GNU C++ library.
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LLVM/Clang based environments:
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Only uses LLVM tools, LLD as a linker, LIBC++ as a C++ standard library.
Also note the following from the README.md file at
https://github.com/mstorsjo/llvm-mingwwhich is where Mingw-w64 seems to get it's clang/llvm implementations:
Clang on its own can also be used as compiler in the normal GNU binutils based environments though, so the main difference lies in replacing binutils with LLVM based tools.