Optimize deconstruction of default literal and expression

[jcouv]

Customer scenario

(int i, string s) = default; doesn't need to instantiate default((int, string)) and then access its .Item1 and .Item2. The parts resulting from deconstruction are default(int) and default(string).

Bugs this fixes

Fixes #21232

Although we could technically apply this optimization in 15.8, I think it makes more sense to stack this on top of the "default deconstruction" feature (#25562).

[jcouv]

@dotnet/roslyn-compiler for review. Two-liner change. Thanks

[AlekseyTs]

    (int i, string s) = default;

It would be good to have a test with default of non-tuple type here, or default of a tuple type of different cardinality.

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Refers to: src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenDeconstructTests.cs:8351 in 465ef3d. [](commit_id = 465ef3d, deletion_comment = False)

[AlekseyTs]

LGTM (iteration 1)

[alrz]

(perhaps test (x, y) = default; where x and y are existing locals/parameters)