Flang OpenMP Report Plugin

This plugin parses Fortran files and creates a
YAML report with all the OpenMP constructs and
clauses seen in the file.

The following tests have been modified to be
compatible for testing the plugin, hence why
they are not reused from another directory:

- omp-atomic.f90
- omp-declarative-directive.f90
- omp-device-constructs.f90

The plugin outputs a single file in the same
directory as the source file in the following format:
`<source-file-name>.yaml`

Building the plugin:
`ninja flangOmpReport`

Running the plugin:
`./bin/flang-new -fc1 -load lib/flangOmpReport.so -plugin flang-omp-report -fopenmp <source_file.f90>`

Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>
Co-authored-by: Stuart Ellis <stuart.ellis@arm.com>

Reviewed By: awarzynski, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D109890