[Coroutines] Define __coro_frame_ty in function scope

Types should be defined in function scope instead of a local lexical scope. Field types should be defined inside in its parent type scope.

We were seeing a type defined in a local scope causing trouble to the dwarf emitter where a context is required to be a funciton scope, a namespace or a global scope.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D104937