Enable Clang-Tidy's `readability-function-size` rule and add a few
`NOLINT` markers to explicitly silence warnings for three functions.
These functions are huge and would IMO benefit from splitting up, but
are hard to without intimate knowledge of the code.
At least by enabling the rule, we can start tweaking the values and
refactoring other functions that bubble up as being too long/complex.
No functional changes.
6414da0381