Add support for C23 interchange float types (`_Float32`, `_Float64`, `_Float128`, ...)

[tgross35]

Newer versions of C allow the following relevant interchange types:

• _Float16: 16-bit IEEE floating point numbers
• _Float32: 32-bit IEEE floating point numbers
• _Float64: 64-bit IEEE floating point numbers
• _Float64x: Extended 64-bit IEEE floating point numbers (e.g. x86 f80 long double)
• _Float128: 128-bit IEEE floating point numbers
• _FloatN _Complex
• Suffix F16/f16
• Suffix F32/f32
• Suffix F64/f64
• Suffix F64x/f64x
• Suffix F128/f128

GCC currently supports all of these. Clang currently supports only _Float16, but should add the rest.

Link to the draft: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2601.pdf
Link to demo: https://clang.godbolt.org/z/5xbq9919z

C++ also supports the same suffixes for the std types, https://en.cppreference.com/w/cpp/types/floating-point

It sounds like these can't simply be redefines of float and double, see #50110 (cc @AaronBallman)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Trevor Gross (tgross35)

GCC supports `_Float32` and `_Float64` in both C and C++, with the associated `F32`, `F64`, and `F128` literal suffixes. Clang should add support for these to be compatible.

It sounds like these can't simply be redefines of float and double, see #50110 (cc @AaronBallman)

[pinskia]

Note this is not exactly a GNU extension but for compatibility with GCC (via C23 defined extended floating point types).

[tgross35]

Yes thanks, I updated the top post to give more context. It seems like GCC does not restrict these types to only --std=c23.