[llvm] [libc] [libc][NFC] Fix "type qualifiers ignored on cast result type" GCC warning (PR #78509)

[Nick Desaulniers via llvm-commits]

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@@ -9,6 +9,8 @@ add_fp_unittest(
     dyadic_float_test.cpp
   DEPENDS
     libc.src.__support.FPUtil.dyadic_float
+  COMPILE_OPTIONS
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nickdesaulniers wrote:

consider adding a comment why this is necessary

https://github.com/llvm/llvm-project/pull/78509