Unused variable prevents compilation about luau HOT 5 CLOSED

It's interesting that this isn't silenced by this:

    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
        list(APPEND LUAU_OPTIONS -Wno-unused) # GCC considers variables declared/checked in if() as unused
    endif()

Maybe our check for the compiler here has to be tweaked to cover MinGW?
Regardless it's something we should fix :)

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The odd part is that even without -Wno-unused, clang-10 doesn't warn on this. This might be new in clang-11. Either way, we'll fix the error - we actually intended to keep clang not using the -Wno-unused warning, but the Makefile used for the CI build right now passes it anyway, which we'll fix as well.

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Oh I just realized this is Emscripten. CMAKE_CXX_COMPILER_ID is probably "Emscripten" or thereabouts. Maybe we should use CMAKE_CXX_COMPILER_IS_GNUCXX instead.

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Oh sorry, I should've mentioned!

from luau.

@zeux CMAKE_CXX_COMPILER_ID indicates that the compiler is actually Clang. A temporary fix would be checking if CMAKE_CXX_COMPILER_ID is equal to "GNU" or "Clang". I have no idea what other compilers would have a similar issue to this (although this fix is a very slippery slope to take).

https://cmake.org/cmake/help/latest/variable/CMAKE_COMPILER_IS_GNUCXX.html
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html

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