[flang][examples] Add missing CMake dependencies

Currently, everything that includes "flang/Parser/parse-tree.h" in Flang
depends on the `gen_acc` and `gen_omp` CMake targets (these targets
generate include files that are used in "parse-tree.h"). The examples in
Flang do use this header file and hence also depend on
`gen_acc`/`gen_omp`. This patch updates relevant CMake scripts
accordingly.

I've also taken the liberty to rename some of the example files so that
their names follow LLVM's coding guidelines.

Differential Revision: https://reviews.llvm.org/D117016