Migrate Declarators to use the List API

After this change all nodes that have a delimited-list are using the
`List` API.

Implementation details:
Let's look at a declaration with multiple declarators:
`int a, b;`
To generate a declarator list node we need to have the range of
declarators: `a, b`:
However, the `ClangAST` actually stores them as separate declarations:
`int a   ;`
`int    b;`
We solve that by appropriately marking the declarators on each separate
declaration in the `ClangAST` and then for the final declarator `int
b`, shrinking its range to fit to the already marked declarators.

Differential Revision: https://reviews.llvm.org/D88403