Explicit any type in for..in loop

For the reason of consistency, it should be possible to explicitly annotate the iterator variable at least with the "any" type in a for..in loop.

*Edit from @sandersn: Fixes should apply to JSDoc too: https://github.com/microsoft/TypeScript/issues/43756, and perhaps allow annotations on `for .. of` too*

The issue names no source files or tests. Start by locating the parser and checker paths for `for..in` iterator declarations, then review the related JSDoc issue #43756 and existing tests for loop-variable annotations. Done means explicit `any` is accepted consistently in the requested `for..in` syntax, with the JSDoc case addressed as specified.

Written by the indexing model from the issue text.