Log Message:
include-what-you-use: updated to 0.20

IWYU 0.20

[iwyu] Support IWYU pragma: export for forward-declarations
[iwyu] Silently break cycles in mappings instead of crashing
[iwyu] Require full type inside typeid()
[iwyu] Improve template reporting and resugaring
[iwyu] Improve reporting of explicit template instantiations
[iwyu] Fix a few crashers
[iwyu] Improve logging (many small fixes)
Abandon python2 for scripts in favor of python3