[AutoDiff] Fix member loading for AD-generated structs/enums.
Fix `IterableDeclContext::loadAllMembers` for structs/enums generated during
differentiation transform.
Previously, this patch disabled `loadAllMembers` for AD-generated structs/enums:
https://github.com/apple/swift/commit/7e89bee39bfda4624f04dcc2c8d53599fbde6191
This caused a SIL verification failure for
test/AutoDiff/autodiff_generated_decl_member_loading.swift because enum members
were not loaded.
Now, `loadAllMembers` is re-enabled. Other necessary fixes:
- Generated structs/enums are set to implicit.
- Since implicit declarations are not printed via `-emit-sil`, data structure
generation tests now rely on `-Xllvm -debug-only=differentiation`.
- Remove calls to `NominalTypeDecl::setBraces`.
- Strategically set `SourceLoc` for `EnumElementDecl` and `EnumCaseDecl` to
prevent duplicate enum cases from printing.
Resolves TF-805.
