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.
