[Binary] Add iterator to the OffloadBinary string maps

The offload binary contains internally a string map of all the key and
value pairs identified in the binary itself. Normally users query these
values from the `getString` function, but this makes it difficult to
identify which strings are availible. This patch adds a simple const
iterator range to the offload binary allowing users to iterate through
the strings.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D127774