This implements the flag proposed in RFC http://lists.llvm.org/pipermail/cfe-dev/2020-August/066437.html.
The goal is to add a way to override the default target C++ ABI through a compiler flag. This makes it easier to test and transition between different C++ ABIs through compile flags rather than build flags.
In this patch:
- Store -fc++-abi= in a LangOpt. This isn't stored in a CodeGenOpt because there are instances outside of codegen where Clang needs to know what the ABI is (particularly through ASTContext::createCXXABI), and we should be able to override the target default if the flag is provided at that point.
- Expose the existing ABIs in TargetCXXABI as values that can be passed through this flag.
- Create a .def file for these ABIs to make it easier to check flag values.
- Add an error for diagnosing bad ABI flag values.