make the whole interface non-throwing

[firewave]

While adding the @throws documentation to the code in #600 I realized there is nonly a single function in the interface which is throwing explicitly (simplecp::characterLiteralToLL()). We should probably be consistent and make this function non-throwing as well.

To enforce that the interface does not throw we should also try to mark functions as noexcept. But we should wait with this until there's a tool-driven way to validate this (i.e. llvm/llvm-project#168324).

[firewave]

Actually simplecpp::preprocess() can throw std::runtime_error. I missed that when adding the documentation.

[firewave]

Actually simplecpp::preprocess() can throw std::runtime_error. I missed that when adding the documentation.

That is actually a bug and is being tracked via #616.