[libc++] [docs] Mark LWG3055 as complete. Use string_view instead of string in...

[libc++] [docs] Mark LWG3055 as complete. Use string_view instead of string in path::operator+=(ECharT).

The issue didn't change the behaviour which is tested in libcxx/test/std/input.output/filesystems/class.path/path.member/path.concat.pass.cpp.

The change to use string_view instead of string is not strictly necessary.

<filesystem> was added in commit 998a5c88 (Implement <filesystem>).

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D92731