Preserve constrained interface setters through spills

[DavidObando]

Summary

• preserve constrained type-parameter/interface metadata when CLR property assignments are rebuilt by synchronous or async spill lowering
• keep ordinary interface receivers on the existing non-constrained path
• cover looped object initializers, ordinary receivers, and awaited initializer values with runtime + ILVerify tests

Validation

• focused regressions: 3 passed (runtime and ILVerify)
• Core.Tests: 6,691 passed, 1 skipped
• Compiler.Tests: 3,404 passed
• all suites run sequentially with MSBUILDDISABLENODEREUSE=1

Oahu proof

• cycle-13 Oahu.Core.dll baseline: 64 ILVerify errors, 2 in BookLibrary::AddPersons
• rebuilt with this compiler: 62 ILVerify errors, 0 in BookLibrary::AddPersons
• fixed IL now emits ldloca + constrained. !!TPerson before both interface setters; fingerprint sha256:4d566c0d76371f805c1e11e8847eaa0f6c90365b740e98ff648d9189e76f30b2 is absent

Deferred work

• none for gsc emitter: constrained generic interface setter in object initializer omits receiver boxing #2741; the remaining 62 Oahu ILVerify findings are unrelated cycle-13 roots

Fixes #2741