CA-054 9.7.2 [enum.udecl] Change grammar

[wg21bot]

Especially given the proposed resolution to CWG2621, https://cplusplus.github.io/CWG/issues/2621.html, it is rather glaring that "using enum" does not follow the lookup rules of elaborated-enum-specifiers (allows typedefs naming enum types) but is subject to syntactic restrictions for elaborated-enum-specifiers (does not allow for typedefs that are alias template specializations).

Change the grammar from:

using elaborated-enum-specifier ;

to:

using enum nested-name-specifieropt identifier ;
using enum nested-name-specifieropt simple-template-id ;

[jensmaurer]

CWG 2022-11-09: Accepted with Modifications. See CWG2621 for details.