[clang-tidy] Constness checks

[j-stephan]

This PR includes the checks that deal with correct constness. Voting will end on 17 November 2020, 18:00 HZDR time.

Included checks:

• Flag postfix operators (x++ / x--) if their return type is not const (Documentation)
• Flag misplaced const on pointer typedefs (Documentation)
• Don't make parameters const that are neither pointers nor references (Documentation)
• Don't return const if the value is neither a pointer nor a reference (Documentation)
• Make parameters that are pointers const if possible (Documentation)

Vote

Check	Enable	Disable
Postfix operator must be const	1	0
Misplaced const	1	0
Avoid const parameters	1	0
Avoid const return types	1	0
Use const pointer parameters	1	0

[bernhardmgruber]

Check	Enable	Disable	Comment
Postfix operator must be const	1	1	It is always weird to return by const value. This could also prevent move optimizations. And it contradicts the Avoid const return types check.
Misplaced const	2	0
Avoid const parameters	2	0
Avoid const return types	2	0
Use const pointer parameters	2	0

[sbastrakov]

Check	Enable	Disable	Comment
Postfix operator must be const	1	2	I agree with Bernhard, but this is also rather rare I think
Misplaced const	3	0
Avoid const parameters	3	0	Note: as the linked docs say, it applies only to function declarations, not definitions (which is good)
Avoid const return types	3	0
Use const pointer parameters	3	0

[j-stephan]

Please merge this!