Add explicit aria-label to placeholder-only inputs

[patrickhlauke]

While placeholder is nominally valid per spec
http://rawgit.com/w3c/html-api-map/master/index.html#accessible-name-and-description-calculation
it is inelegant, a fallback, and not supported in ios/safari/voiceover
and android/chrome/talkback, to name a few combinations

aria-describedby is also not really the right tool for providing a
name/label equivalent, so remove that from the list of possible
alternative methods / clarify how it can be used

closes #22302

[patrickhlauke]

xref w3c/html-aam#84 - in theory, IF iOS/VO and Android/Chrome/TalkBack fix their bugs, this PR's main change (adding extra aria-label) may be superfluous...but right here, right now, it's needed for these platforms (and even once fixed it's not actually wrong to have aria-label explicitly, just perhaps a bit "belt and braces" redundancy)

[patrickhlauke]

adding to this: since I raised UA inconsistencies, it's now been officially decided to remove placeholder from the official "accessible name calculation" algorithm in HTML. user agents may still do this, but it's outside of spec, and won't be guaranteed to work cross-browser. in short: this PR was the right call w3c/html-aam#87