Simplify expression type to fix build break

[weswigham]

Fixes the post-LKG build break introduced by the changes in #41891 by simplifying the expression type in use by widening the type of a ternary to string, rather than a union of literal types.

[ahejlsberg]

Yeah, that's a nasty little demonstration of exponential growth. It's interesting because the template literal expression that blows up actually only has two possible values. We could perhaps consider a template literal type representation where multiple holes can be marked as having the same origin. Such shared origin holes would be iterated in lockstep when we produce the normalized template literal union type. But for now, we should definitely merge this PR.