[Version 9.0] Feature support for target-typed conditional expression

[RexJaeschke]

This PR is my formal version of the corresponding MS spec. Previously, I created PR #1071 to address this, but when Bill went to push that work into the new v9 process, he found it badly compromised. As such, that PR will be retired and this one will be used in its place.

12.18 Conditional operator has an open issue; see the following words in that section: placeholder for words somehow referring to “12.6.4.5 Better conversion from expression.”

Here are the details:

Mail to Bill on 2024-03-18: The MS spec suggests the text for “12.6.4.5 Better conversion from expression” be augmented; OK. However, “12.18 Conditional operator“ does not reference that section; instead, it points to “12.6.3.15 Finding the best common type of a set of expressions.” So, I don’t see how the proposed changes to “better conversion” will be seen in the context of “best common type.” Unless these two are somehow linked and I’m not seeing where.

Bill’s reply on 2024-04-01: I think the conditional expression section should reference both sections, as sometimes the conditional expression applies when there is no best common type.