[libc++] [LIBCXX-DEBUG-FIXME] Fix an iterator-invalidation issue in string::assign.

This appears to be a bug in our string::assign: when assigning into
a longer string, from a shorter snippet of itself, we invalidate
iterators before doing the copy. We should invalidate them afterward.
Also drive-by improve the formatting of a function header.

Differential Revision: https://reviews.llvm.org/D101675