Diagnose problematic uses of constructor/destructor attribute (#67673)

Functions with these attributes will be automatically called before
main() or after main() exits gracefully. In glibc environments, the
constructor function is passed the same arguments as main(), so that
signature is allowed. In all other environments, we require the function
to accept no arguments and either return `void` or `int`. The functions
must use the C calling convention. In C++ language modes, the functions
cannot be a nonstatic member function, or a consteval function.

Additionally, these reuse the same priority logic as the init_priority
attribute which explicitly reserved priorty values <= 100 or > 65535. So
we now diagnose use of reserved priorities the same as we do for the
init_priority attribute, but we downgrade the error to be a warning
which defaults to an error to ease use for implementers like compiler-rt
or libc.

This relands a633a376 with fixes.