C++: Fix crashes with the pattern "enum class operator A"

Parser parsed the pattern as an enum declaration the name of
which is a conversion operator. Add check to disallow keywords
after enum-key (enum, enum class, enum struct).
Add tests tst_AST::enumDeclaration and
invalidEnumClassDeclaration.

Task-number: QTCREATORBUG-15341
Change-Id: Ia037f00184c1d7e5b0374f39331bb6748f8d90b1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>