[llvm][ADT] Overload output stream operator `<<` for `StringMapEntry` and `StringMap`.

Printing support enables the production of more useful error messages in unit testing e.g. when using matchers such as `UnorderedElementsAre()` to inspect the contents of a `StringMap`.

Reviewed By: gribozavr2, sgatev, ymandel

Differential Revision: https://reviews.llvm.org/D132747