Support extra compiler flags in single file tests

[afd]

Extra flags, such as -std=c++20, can be passed to Dredd on the command line, e.g.:

./third_party/clang+llvm/bin/dredd --mutation-info-file temp.json blah.cc -- -std=c++20

For single file tests, if we would like to be able to test examples that use special features, we would need a way to specify such options.

An example feature that would need this is constinit.

@JamesLee-Jones for info.