[libc++][syncbuf] Implements LWG3253.

Note the tests already tested this behaviour and the wording change is NFC.

Implements
- LWG3253 basic_syncbuf::basic_syncbuf() should not be explicit

Author: mordante

libcxx/docs/Status/Cxx20Issues.csv

@@ -172,7 +172,7 @@
 "`3221 <https://wg21.link/LWG3221>`__","Result of ``year_month``\  arithmetic with ``months``\  is ambiguous","Belfast","|Complete|","8.0"
 "`3235 <https://wg21.link/LWG3235>`__","``parse``\  manipulator without abbreviation is not callable","Belfast","",""
 "`3246 <https://wg21.link/LWG3246>`__","What are the constraints on the template parameter of ``basic_format_arg``\ ?","Belfast","","","|format|"
-"`3253 <https://wg21.link/LWG3253>`__","``basic_syncbuf::basic_syncbuf()``\  should not be explicit","Belfast","",""
+"`3253 <https://wg21.link/LWG3253>`__","``basic_syncbuf::basic_syncbuf()``\  should not be explicit","Belfast","|Complete|","20.0"
 "`3245 <https://wg21.link/LWG3245>`__","Unnecessary restriction on ``'%p'``\  parse specifier","Belfast","","","|chrono|"
 "`3244 <https://wg21.link/LWG3244>`__","Constraints for ``Source``\  in |sect|\ [fs.path.req] insufficiently constrainty","Belfast","",""
 "`3241 <https://wg21.link/LWG3241>`__","``chrono-spec``\  grammar ambiguity in |sect|\ [time.format]","Belfast","|Complete|","16.0","|chrono| |format|"

libcxx/include/syncstream

@@ -46,7 +46,9 @@ namespace std {
         using streambuf_type = basic_streambuf<charT, traits>;
 
         // [syncstream.syncbuf.cons], construction and destruction
-        explicit basic_syncbuf(streambuf_type* obuf = nullptr)
+        basic_syncbuf()
+          : basic_syncbuf(nullptr) {}
+        explicit basic_syncbuf(streambuf_type* obuf)
           : basic_syncbuf(obuf, Allocator()) {}
         basic_syncbuf(streambuf_type*, const Allocator&);
         basic_syncbuf(basic_syncbuf&&);
@@ -253,7 +255,10 @@ public:
 
   // [syncstream.syncbuf.cons], construction and destruction
 
-  _LIBCPP_HIDE_FROM_ABI explicit basic_syncbuf(streambuf_type* __obuf = nullptr)
+  _LIBCPP_HIDE_FROM_ABI basic_syncbuf()
+      : basic_syncbuf(nullptr) {}
+
+  _LIBCPP_HIDE_FROM_ABI explicit basic_syncbuf(streambuf_type* __obuf)
       : basic_syncbuf(__obuf, _Allocator()) {}
 
   _LIBCPP_HIDE_FROM_ABI basic_syncbuf(streambuf_type* __obuf, _Allocator const& __alloc)