Accidental token pasting

[paulross]

The preprocessor can be coerced into accidentally pasting tokens to create significant other tokens. For example:

#define PLUS +
+PLUS

This should produce + + not ++.

There are some xfailing tests in tests/unit/test_core/test_PpLexer.py. Search for @pytest.mark.xfail(reason='Need to fix accidental token pasting.')

Some links:

• GCC documentation https://gcc.gnu.org/onlinedocs/cppinternals/Token-Spacing.html
• CPP Internals: http://wwwcdf.pd.infn.it/localdoc/cppinternals.ps.gz