Arguments in `SubscriptExpr` are visited since the recent `ArgumentList`
refactoring, but were being added to the containing `CallExpr`. Add a
node for the `SubscriptExpr` itself so that its argument is added there
instead of the `CallExpr`.
Also remove `key.nameoffset` and `key.namelength` from the response when
both are 0 to match the rest of the offsets and lengths.
Resolves rdar://85412164.
While the decls being printed for header file generated interfaces were mapped
from the top-level clang decls in that file, the Swift decls they correspond to
may not be top-level. E.g. top-level functions in the header file can be mapped
to property accessors on the Swift side, which were being printed simply as
"get" at the top level.
This updates header interface generation to map each decl to its top-level decl
before printing.
Resolves rdar://problem/63409659
