We now call Preprocessor::process_and_lex() and pass the result to the
parser.
Doing the lexing in the preprocessor will allow us to maintain the
original position information of tokens after substituting definitions.
For each .cpp file in the test suite data, there is a .ast file that
represents the "known good" baseline of the parser result.
Each .cpp file goes through the parser, and the result of
invoking `ASTNode::dump()` on the root node is compared to the
baseline to find regressions.
We also check that there were no parser errors when parsing the .cpp
files.
