make type contexts mostly shallow

[metagn]

TypeDeclContext is used to track if:

• nominal types can be declared, i.e. the type expression is the direct value of a type symbol declaration
• the type . is valid, either as void for return types or the lack of a constraint for generic params
• the standalone symbols array/range/set can be turned into typeclasses of the respective type, checks for a constraint context but should probably be separated

These are mostly shallow checks, so contexts are no longer propagated in general. The only cases where they're still propagated are:

• ParX
• or/and types, these could also use InGenericConstraint or some other typeclass context too
• inlined type symbols (not symbols produced by semInvoke which use InLocalDecl which could be changed too but in another PR)

The unused fields of TypeDeclContext are removed in case they weren't meant to be shallow.