[basic.scope.scope] Replace "warning" with "diagnostic message"

[eisenwave]

We don't usually say "warning", not even in https://eel.is/c++draft/cpp.error#:preprocessing_directive,warning. Everywhere else, the standard talks about diagnostic messages.

[frederick-vs-ja]

Hmm, the standard wording is currently saying "warning" for many times:

• diagnostic message: [dcl.pre], [dcl.fct.def.delete], [dcl.attr.deprecated], [defns.diagnostic], [defns.undefined], [intro.compliance.general], [cpp.error]
• warning: [basic.scope.scope], [dcl.attr.fallthrough], [dcl.attr.unused], [dcl.attr.nodiscard], [dcl.attr.noreturn], [diff.expr], [diff.stmt]