Make sure to use normalized ty for unevaluated const in default struct value
This cleans up the way that we construct the `mir::Const::Unevaluated` for default struct values. We were previously using `from_unevaluated`, which doesn't normalize the type, and is really only used for inline assembly. Other codepaths (such as `ExprKind::NamedConst`) use the type from the body.
Also, let's stop using `literal_operand`, which also is really not meant for calls other than for literal comparisons in pattern lowering.
Also move all of the tests to a separate subdirectory so they don't need to have the same prefix on all the test files.
Fixes#134298
r? estebank or reassign
050e0cc6eb