pp-trace is a tool for displaying preprocessor activity by means of the PPCallbacks interface. Its primary reason for existence is for testing the PPCallbacks mechanism, but it also might be useful as a tool for understanding preprocessor activity (as an alternative to clang's -P option for looking at preprocessor output).
This is a first cut, for proof of concept, with only two tests for demonstration purposes.
It supports two output formats, YAML and a generic format. Originally I just put in the YAML, but I felt it was kind of verbose, so I added the generic format as a default.
Please see the file comments for details.
Documentation and more tests will follow in a later checkin.