Add a warning, flags and pragmas to limit the number of pre-processor tokens in a translation unit

See
https://docs.google.com/document/d/1xMkTZMKx9llnMPgso0jrx3ankI4cv60xeZ0y4ksf4wc/preview
for background discussion.

This adds a warning, flags and pragmas to limit the number of
pre-processor tokens either at a certain point in a translation unit, or
overall.

The idea is that this would allow projects to limit the size of certain
widely included headers, or for translation units overall, as a way to
insert backstops for header bloat and prevent compile-time regressions.

Differential revision: https://reviews.llvm.org/D72703