c8ef 332ac18e31 [clang] constexpr built-in abs function. (#112539) ...

According to [P0533R9](https://wg21.link/P0533R9), the C++ standard
library functions corresponding to the C macros in `[c.math.abs]` are
now `constexpr`.

To implement this feature in libc++, we must make the built-in abs
function `constexpr`. This patch adds the implementation of a
`constexpr` abs function for the current constant evaluator and the new
bytecode interpreter.

It is important to note that in 2's complement systems, the absolute
value of the most negative value is out of range. In gcc, it will result
in an out-of-range error and will not be evaluated as constants. We
follow the same approach here.

2024-10-18 19:03:50 +08:00

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