[Clang] Refactor `__attribute__((assume))` (#84934)

This is a followup to #81014 and #84582: Before this patch, Clang 
would accept `__attribute__((assume))` and `[[clang::assume]]` as 
nonstandard spellings for the `[[omp::assume]]` attribute; this 
resulted in a potentially very confusing name clash with C++23’s 
`[[assume]]` attribute (and GCC’s `assume` attribute with the same
semantics).

This pr replaces every usage of `__attribute__((assume))`  with 
`[[omp::assume]]` and makes `__attribute__((assume))` and 
`[[clang::assume]]` alternative spellings for C++23’s `[[assume]]`; 
this shouldn’t cause any problems due to differences in appertainment
and because almost no-one was using this variant spelling to begin
with (a use in libclc has already been changed to use a different
attribute).