[clang] [clang] Implement CTAD for type alias template. (PR #77890)

[Erich Keane via cfe-commits]

erichkeane wrote:

>We could revert the whole patch when things are broken (and it is what we usually do). But I'm not sure revert/reland a big patch back and forth is a good idea.

This is effectively what we did repeatedly with the deferred concepts patch and IMO, was a really nice healthy way of doing it.  If you organize your commits (much easier on github than it was on Phab), you can present JUST the changes between revisions/fixes and get a really nice revert/fix/release cycle going.

I'd encourage just enabling by default (to avoid losing the test breadth that comes along with it), and reverting if there are sufficiently 'bad' breakages.  

Note that you'd not have to revert for all 'bugs', just those that break existing code.  We can hold off on setting the Feature Test Macro until we're comfortable with it though.

https://github.com/llvm/llvm-project/pull/77890