[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 998a5c8 (Implement <filesystem>).

Reviewed By: #libc, ldionne

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

Author: mkurdej

libcxx/docs/Cxx2aStatusIssuesStatus.csv

@@ -143,7 +143,7 @@
 "`3182 <https://wg21.link/LWG3182>`__","Specification of ``Same``\  could be clearer","Kona","",""
 "","","","",""
 "`2899 <https://wg21.link/LWG2899>`__","``is_(nothrow_)move_constructible``\  and ``tuple``\ , ``optional``\  and ``unique_ptr``\ ","Cologne","",""
-"`3055 <https://wg21.link/LWG3055>`__","``path::operator+=(*single-character*)``\  misspecified","Cologne","",""
+"`3055 <https://wg21.link/LWG3055>`__","``path::operator+=(*single-character*)``\  misspecified","Cologne","|Complete|","7.0"
 "`3158 <https://wg21.link/LWG3158>`__","``tuple(allocator_arg_t, const Alloc&)``\  should be conditionally explicit","Cologne","",""
 "`3169 <https://wg21.link/LWG3169>`__","``ranges``\  permutation generators discard useful information","Cologne","",""
 "`3183 <https://wg21.link/LWG3183>`__","Normative permission to specialize Ranges variable templates","Cologne","",""

libcxx/include/filesystem

@@ -940,9 +940,8 @@ public:
   template <class _ECharT>
   typename enable_if<__can_convert_char<_ECharT>::value, path&>::type
   operator+=(_ECharT __x) {
-    basic_string<_ECharT> __tmp;
-    __tmp += __x;
-    _PathCVT<_ECharT>::__append_source(__pn_, __tmp);
+    _PathCVT<_ECharT>::__append_source(__pn_,
+                                       basic_string_view<_ECharT>(&__x, 1));
     return *this;
   }