Inline basic_string::erase for fastpath where __n == npos

Summary:
This change checks for the case where people want to erase a string to the end, i.e., __n == npos, and inlines the call if so.

This also demonstrates keeping the ABI intact for V1, but inlining the erase() method for unstable.

Reviewers: EricWF, mclow.lists, ldionne

Reviewed By: EricWF, ldionne

Subscribers: smeenai, dexonsmith, christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D73743