[libc++] Decrease instantiation cost of __constexpr_memmove (#125109)

Using `if constexpr` in `__constexpr_memmove` makes the instantiation
three times faster for the same type, since it avoids a bunch of class
instantiations and SFINAE for constexpr support that's never actually
used. Given that `__constexpr_memmove` is used quite a bit through
`std::copy` and is instantiated multiple times when just including
`<__string/char_traits.h>` this can provide a nice compile time speedup
for a very simple change.

0 files changed, 0 insertions, 0 deletions