[PATCH 0/5, OpenACC] Add support for Fortran optional arguments in OpenACC

[Kwok Cheung Yeung]

This patchset allows the use of Fortran optional arguments in OpenACC 
programs in accordance with section 2.17 of the OpenACC 2.6 specification.

These patches were originally posted at 
https://gcc.gnu.org/ml/gcc-patches/2019-01/msg01750.html for the OG8 
branch. This version is targeted at trunk.

Tested on x86_64 with offloading to a Nvidia Tesla K20c card.

Okay to apply to trunk?

Thanks

Kwok