[clang-tidy] readability-function-size: fix nesting level calculation

Summary:
A followup for D32942.

Malcolm Parsons has provided a valid testcase that the initial version of the check complained about nested `if`'s.
As it turns out, the culprit is the **partially** un-intentional `switch` fallthrough.
So rewrite the NestingThreshold logic without ab-using+mis-using that switch with fallthrough, and add testcases with nested `if`' where there should be a warning and shouldn't be a warning. This results in a cleaner, simpler code, too.

I guess, now it would be actually possible to pick some reasonable default for `NestingThreshold` setting.

Fixes PR33454.

Reviewers: malcolm.parsons, alexfh

Reviewed By: malcolm.parsons

Subscribers: sbenza, xazax.hun, cfe-commits, aaron.ballman

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34202

llvm-svn: 305554