Port F18 semantics/test_errors.sh to equivalent to Clang -verify

[RichBarton-Arm]

After #941 is completed, F18 will contain a directory called test containing regression tests for the F18 compiler driven by the LLVM lit testing framework. LLVM projects tend also to use FileCheck for such tests whilst the F18 tests will still use custom bash scripts instead. During the review for #934 we identified that FileCheck may not be the best option for porting the tests that use semantics/test_errors.sh. Instead we proposed that if Flang had an equivalent feature to Clang's -verify option then test_errors.sh tests would port well to that.

This task is to investigate if a flang -verify option is feasible and if so implement it and port semantics/test_errors.sh to using it. If it is not feasible, then investigate what a FileCheck version of semantics/test_errors.sh would look like so we can consider that option again.

Note that the rest of the test scripts are expected to port to using FileCheck under #996

[sscalpone]

What is the advantage to using an internal -verify implementation vs an external script?