[libc++] Introduce a setting to remove fstream from the library

This allows porting the library to platforms that are able to support
<iostream> but that do not have a notion of a filesystem, and where it
hence doesn't make sense to support std::fstream (and never will).

Also, remove reliance on <fstream> in various tests that didn't
actually need it.

Differential Revision: https://reviews.llvm.org/D138327