Clarify [EventHandler] tag helper discovery logic and avoid exception

[DustinCampbell]

This fixes an issue that @tmat pointed out to me over email. In a recent change (#10720), I added a call to Assumed.Unreachable() when [EventHandler] tag helper discovery encounters an attribute with invalid constructor attributes. However, throwing an exception during tag helper discovery is usually the wrong approach. Normally, if Roslyn symbols aren't in the proper shape during tag helper discovery, Razor will simply not produce a tag helper. (We do support diagnostics for tag helpers, but those are usually reserved for warnings and errors that are related to a tag helper's data that would make it unusable, such as a name containing whitespace.)

It turns out that the "unreachable" condition wasn't actually all that unreachable and @tmat was hitting it while working on hot reload tests. So, I've changed the code to make the success conditions clearer, i.e., the attribute data must match one of the two constructor calls. And, I changed the logic to simply skip [EventHandler] attributes that don't meet the success conditions.

[tmat]

Thanks!

[DustinCampbell]

Thanks everyone!