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.
And fix the diagnostics for __builtin_is_constant_evaluated(). We can be
in a non-constant context, but calling an immediate function always
makes the context constant for the duration of that call.
The new constant interpreter's `clang::interp::InterpState` contains
both `clang::interp::Context` and `clang::ASTContext`. So using `S.Ctx`
and `S.getCtx()` was a bit confusing. This PR rename `getCtx()` to
`getASTContext` to make things more clearer.
Signed-off-by: yronglin <yronglin777@gmail.com>
As per [P0533R9](https://wg21.link/P0533R9), the corresponding C++
`[c.math.fpclass]` standard library functions for the C macros are now
`constexpr`.
The only classification function that wasn't already `constexpr` was
`__builtin_signbit`.
The floating point comparison functions `__builtin_isgreater`,
`__builtin_isgreaterequal`, `__builtin_isless`, `__builtin_islessequal`,
`__builtin_islessgreater` and `__builtin_isunordered` are now
`constexpr`.
The C23 macro `iseqsig` is not currently supported because
`__bulitin_iseqsig` doesn't exist yet (and C++26 is still currently
based on C18).
This also allows them to be constant folded in C, matching the behaviour
of GCC.
