CWG3020 [cpp.cond] Missing specification for __has_cpp_attribute(indeterminate)

[timsong-cpp]

Full name of submitter (unless configured in github; will be published with the issue): Tim Song

Reference (section label): [cpp.cond]

Link to reflector thread (if any):

Issue description:

P2795R5 should, but did not, update the definition of __has_cpp_attribute for the new attribute it added.

Other attributes also have a note or recommended practice on when __has_cpp_attribute should produce zero, but it is not clear if there is anything to say about [[indeterminate]] as we presumably do not want to suggest that a value of zero makes the undefined behavior erroneous instead (cf. [[noreturn]]).

Suggested resolution:

Add to Table [tab:cpp.cond.ha]:

Attribute	Value
indeterminate	202403L