[flang] Upstream partial lowering of GET_ENVIRONMENT_VARIABLE intrinsic

This patch adds partial lowering of the "GET_ENVIRONMENT_VARIABLE" intrinsic
to the backend runtime hook implemented in patches D111394 and D112698.
It also renames the `isPresent` lambda to `isAbsent` and moves it out to
its own function in `Command.cpp`. Corresponding comment fixes for this
are also modified. Lastly it adds the i1 type to
`RuntimeCallTestBash.h`.

Differential Revision: https://reviews.llvm.org/D118984