Port F18 lit tests to using FileCheck or suitable LLVM-alternative

[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.

This task is to evaluate in turn each of the custom scripts and consider if they can be replaced with FileCheck and rewrite the tests accordingly.

The list of test scripts are:

• semantics/test_modfile.sh
• semantics/test_symbols.sh
• semantics/test_any.sh
• semantics/common.sh
• evaluate/test_folding.sh

The tests in semantics that use test_errors.sh will be covered under #996

For each script, the porting order of preference would be:

1. Port to FileCheck
2. Port to some other LLVM-like testing utility or feature
3. Rewrite the bash script to python

[RichBarton-Arm]

Ivan Zechev ported the rest of the tests over to Python - see https://reviews.llvm.org/rGdc453dcf760e6d558da3a4d7fff301baa5f37aba for last commit in the series.