Introduce NvGetIntConverted so IntMap array can warn on converted numeric string keys

Summary: IntMap array is an array with only integer keys, that does not support
converting numeric string keys to integer keys. It instead raises a warning
when such a thing occurs. This diff adds to ArrayFunctions a function to
warn on such occurrences, but all other ArrayKinds will just delegate to
NvGetInt without warning.

Reviewed By: @paroski

Differential Revision: D1428302