[META] opaque type handling

[lcnr]

Opaque types are have some non-trivial backcompat requirements and generally require a lot of care to support correctly.

misc

• instantiating an opaque with another opaque stops being an error #29
• opaque types in the defining scope auto trait candidate #32
• not normalizing tait parameters before checking if it's a defining usage #49
• new solver equates opaque to trait object instead of coercing #147

problematic may-define outside of typeck

• addr2line fails to relate an opaque type #106

nested bodies

• Nested bodies need to have pre-defined opaques #132

normalizing opaque types allows more code to compile

• normalizing opaques in outlives constraints allows more code compile #112

treating opaque as rigid during typeck

• method calls on opaques in defining scope: auto deref steps results in ambiguity  #47
• method resolution can use yet undefined opaques #131
• non-defining uses with non-param args will be break #135
• recursive call results in unconstrained opaque type args #145

proving item bounds during normalization

• normalizing an opaque while proving its item bounds causes overflow errors #143

opaque type with placeholder args

• tests/ui/type-alias-impl-trait/param_mismatch.rs
• tests/ui/type-alias-impl-trait/under-binder.rs
• tests/ui/closures/supertrait-hint-cycle.rs
• surprising error with opaque types #204