This commit adds tests for CGo preprocessing. There are various errors
that can be reported while preprocessing, and they should integrate well
with the compiler (including accurate source location tracking).
Also allow CGo preprocessing to continue after Clang encountered an
error, for a better view of what happened.
When this flag is set, the testdata/*.out.go files will be updated when
they have changed. This is very convenient for updating these files
after the expected output changes.
Of course, the updated output must still be checked for validity.
