The run_thunkgen* helpers now parse generated source code and return its AST
representation, so HasASTMatching helper calls don't each need to redundantly
compile it themselves. This also ensures the generator output actually compiles
in tests where we didn't explicitly check that before.
This also allows printing the full AST of the generator output on test
failures. This must be enabled manually by changing a variable in the ostream
output operator for SourceWithAST.
Tony Wasserka