[libc] make part of libc's unit test framework independent of llvm

This is mostly changing stringref to std::string, outs() to cout,
and small supporting changes. This will make running unit tests possible
on systems that are only grabbing the libc part of llvm.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D91568

GitOrigin-RevId: a200d6cdf0f7eafafead147f2e18e5456ed7bbe1