Resolves https://github.com/dlang/dmd/issues/20434
Allowing this gnu/clang extension actually removes lines of code as
we can unify the parsing and semantics of `_Alignof` and `sizeof`
and have the normal D machinery handle the difference later.
Fixes https://github.com/dlang/dmd/issues/20472
Arrays in C implicitly convert to a pointer to their first member,
so do the implicit conversion when using them in an arrow member lookup.
