[flang][nfc] Fix driver method names overridden by the plugins

After the recent re-factoring of the driver code
(https://reviews.llvm.org/D125007), `ExecuteAction` was renamed as
`executeAction`. This patch updates the examples in Flang accordingly.

If you set `FLANG_BUILD_EXAMPLES` to `On` when building Flang, then the
refactoring from D125007 would have caused build failures for you. This
patch fixes that.

This is fairly straightforward and fixes buildbot failures, so I'm
sending this without a review.