rustc_resolve: don't deny outer type parameters in embedded constants.
This solves a problem noted at https://github.com/rust-lang/rust/issues/29646#issuecomment-300929548, where an associated const default in a trait couldn't refer to `Self` or type parameters, due to inaccuracies in lexical scoping.
I've also allowed "embedded expressions" (`[T; expr]`, `[x; expr]`, `typeof expr`) to refer to type parameters in scope. *However*, the typesystem still doesn't handle #34344.
Fully resolving that issue requires breaking cycles more aggressively (e.g. lazy evaluation), *even* in when the expression doesn't depend on type parameters, to type-check it at all, and then also type-level "constant projections" (in the vein of `{expr}` from const generics).
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