Update the diagnostic behavior of [[noreturn]] in C2x

Post-commit review feedback suggested dropping the deprecated
diagnostic for the 'noreturn' macro (the diagnostic from the header
file suffices and the macro diagnostic could be confusing) and to only
issue the deprecated diagnostic for [[_Noreturn]] when the attribute
identifier is either directly written or not from a system macro.

Amends the commit made in 5029dce4.