Should pmr::memory_resource::allocate(n, a) use bit_ceil(a)?

[Jonathan Wakely]

I should probably adopt Martin's patch from
https://gcc.gnu.org/PR86878 and finish it, so that we get warnings for
invalid non-power-of-two alignments passed to allocation functions,
but since we don't have that ...

Would it be a good idea for pmr::memory_resource::allocate(n, a) to call:
  do_allocate(n, bit_ceil(a))
?

That would ensure that the virtual override of do_allocate is always
called with a valid alignment, even if the user violates the
precondition of memory_resource::allocate.

When the argument is a constant, the compiler can optimize bit_ceil(a)
away if it knows it's already a power of two.

Without this, I'm tempted to put an assertion or do the bit_ceil(a) in
each override of do_allocate, to avoid potential UB caused by invalid
alignment values.