[libc++] Fix initialization of __fill_

`basic_ios` delays initialization of `__fill_` to `widen(' ')` until `fill()` is called. But, `fill(char_type)` is missing this logic, so the fill character does not get initialized to whitespace if `fill(char_type)` is called first. This patch adds this logic to `fill(char_type)`.

Reviewed By: #libc, ldionne, Quuxplusone

Differential Revision: https://reviews.llvm.org/D120751