Enable `clang-tidy`'s `readability-function-cognitive-complexity` check

[eddyashton]

Diff to enable this looks something like this:

diff --git a/.clang-tidy b/.clang-tidy
index c0a984def..941f67e03 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -39,11 +39,16 @@ Checks: >
   -performance-no-int-to-ptr,
   portability-*,
   readability-*,
-  -readability-function-cognitive-complexity,
   -readability-identifier-length,
   -readability-avoid-nested-conditional-operator,
   -readability-convert-member-functions-to-static,
 
+CheckOptions:
+  - key: readability-function-cognitive-complexity.IgnoreMacros
+    value: 'true'
+  - key: readability-function-cognitive-complexity.Threshold
+    value: '50'
+
 WarningsAsErrors: '*'
 HeaderFilterRegex: '?!(3rdparty)'
 FormatStyle:     'file'

I think we definitely want to ignore macros, because our logging macros are measured as extremely complex but in practice don't make the functions harder to read.

The default threshold is 25, which flags a huge number of functions, and I think is a little low. I suggest we start with a higher threshold, such as 50, for at least an initial pass.

A benefit of this (beyond pure readability) should be that we improve the clang-tidy coverage for other checks - such as bugprone-unchecked-optional-access - to silently fail, missing clear errors. By simplifying functions, reducing the scope that these checks need to analyse, we should get better coverage. One frustrating niggle is that it's not clear to me what a "safe" threshold for this is - the "cognitive complexity" does not directly correspond with the flow-analysis complexity that causes these checks to fail, and we don't know what threshold the checks fail at. But these measures are likely correlated, and we can do some work to validate where certain checks are and are not being run.