Xcode clang doesn't understand the -std=c++23 spelling yet, and this
is what CMake's `set(CMAKE_CXX_STANDARD 23)` translates to too.
Unbreaks building with Xcode clang on macOS.
FP contraction is a standard-conforming behavior which allows the
compiler to calculate intermediate results of expressions containing
floating point numbers with a greater precision than the expression type
allows. And in theory, it enables additional optimizations, such as
replacing `a * b + c` with fma(a, b, c).
Unfortunately, it is extremely hard to predict when the contraction will
happen. For example, Clang 17 on x86_64 with the default options will
use FMA only for constant-folded non-constexpr expressions. So, in
practice, FP contraction leads to hard-to-find bugs and inconsistencies
between executables compiled with different toolchains or for different
OSes. And we had two instances of this happening last week.
Since we did not ever used -mfma on x86_64, this patch can only possibly
regress performance on Apple ARM devices, where FMA is enabled by
default. However, this regression will likely be negligible since the
difference would be one additional add instruction, which would be then
likely executed in parallel with something else.
This makes it possible to specify (instead of is_debug) a more
specialized is_debug_lldb or is_debug_gdb, so that clang outputs the
proper symbols. (For lldb this fixes issues with formatting by
setting -fstandalone-debug)
