[flang] Examples of Composite and Combined constructs (flang-compiler/f18#558)

* Examples of Composite and Combined constructs

2.8.3 Loop SIMD Construct
2.11.1 Parallel Loop Construct
2.11.3 Parallel Loop SIMD Construct

Not in the implementation:
2.8.3 DO SIMD: A list item may appear in a linear or firstprivate clause but not both

Original-commit: flang-compiler/f18@75d9c1a081474c02a3e106ac13924134e06793ca
Reviewed-on: https://github.com/flang-compiler/f18/pull/558