[clang-tidy] readability-function-size: add NestingThreshold param.

Summary:
Finds compound statements which create next nesting level after `NestingThreshold` and emits a warning.
Do note that it warns about each compound statement that breaches the threshold, but not any of it's sub-statements, to have readable warnings.

I was able to find only one coding style referencing nesting:
  - https://www.kernel.org/doc/html/v4.10/process/coding-style.html#indentation
     > In short, 8-char indents make things easier to read, and have the added benefit of warning you when you’re nesting your functions too deep.

This seems too basic, i'm not sure what else to test. Are more tests needed?

Reviewers: alexfh, aaron.ballman, sbenza

Reviewed By: alexfh, aaron.ballman

Subscribers: xazax.hun

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D32942

llvm-svn: 305082