Implement most RVO cases in glue layer

[limepoutine]

#22412, part 3

This PR enables direct construction in ConstructExp for any LHS and most RHS expressions, with two exceptions:

• v = T(S(1)) doesn't emplace S(1) (a second PR is pending)
• v = T(a: S(b: U(1)).a.b doesn't fold to v = U(1) (implementing this doesn't appear to be of much benefit)

Also fixes the bug that a = b = S(1) fails to call the copy constructor of S. Note the removed tests are fully covered by new ones.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#22526"

[limepoutine]

Is it the test runners ran out of memory twice in a row, or is it I introduced a bug? Needs investigation anyway.