[dcl.attr.noreturn]/3 Replace `[[noreturn]]` with `noreturn` for consistency

[BlowingWind314]

[dcl.attr.noreturn]/3 says

Recommended practice: Implementations should issue a warning if a function marked [[noreturn]] might return. The value of a has-attribute-expression for the noreturn attribute should be 0 unless the implementation can issue such warnings.

It uses double brackets to enclose the attribute.

However, the enclosing double brackets are not used in all other similar contexts in [dcl.attr].

[jensmaurer]

Agreed. Patch welcome, but please also fix

"Reading from an uninitialized variable that is marked [[indeterminate]] can cause undefined behavior."