Test with C++ multi-line string literals

[pgoodman]

The way the Run method works is that each token is put into ASTImpl::preprocessed_code on its own line, so that we can map from source location line number back to index in ASTImpl::tokens. This means fiddling with embedded newlines in tokens, and thus introduces flakiness. My guess is that this approach is going to fall over when we want to handle C++ multiline string literals. Regardless, an easier approach would be to append unadulterated token data into ASTImpl::preprocessed_code, and build up a vector of token begin or token end indexes, for binary searching later. Alternatively, we could try some kind of binary search inside of ASTImpl::tokens, as they have data_offset, but that will require being clever, as the data_offset has an overloaded meaning.