Add clang-tidy to buildsystem checks

[kev946]

Fixes #1060

[TheJJ]

Thank you! I think clang-tidy needs to be aware of the build system (https://blog.kitware.com/static-checks-with-cmake-cdash-iwyu-clang-tidy-lwyu-cpplint-and-cppcheck/ and http://mariobadr.com/using-clang-tidy-with-cmake-36.html).

What we need to think about is how/when we merge this. Because previously, we have to fix all the issues it finds. And we should never ever have a non-passing CI state.
So my proposal is to not enable the check by default, so we can gradually fix all the problems and after that, we enable it so that Kevin fails.

[simonsan]

I want to let this here:
https://github.com/KDE/clazy

clazy is a compiler plugin which allows clang to understand Qt semantics. You get more than 50 Qt related compiler warnings, ranging from unneeded memory allocations to misusage of API, including fix-its for automatic refactoring.

Here is a list with checks:
https://github.com/KDE/clazy#list-of-checks

[simonsan]

Which of these checks do we want to focus on?
I would propose a selection of the following with another clang-tidy-parameter stating explicitly -extra-arg=-std=c++17. clang-diagnostic-* gives probably the most warnings, so we should add these checks in the end when we are fine with the rest.

EDIT1: Excluded some spammy commands from the list.

cppcoreguidelines-* | Checks related to C++ Core
clang-analyzer-* | Clang Static Analyzer checks.
performance-* | Checks that target performance-related issues.
portability-* | Checks that target portability-related issues that don’t relate to any particular coding style.
misc-* | Checks that we didn’t have a better category for.
modernize-* | Checks that advocate usage of modern (currently “modern” means “C++11”) language constructs.
hicpp-* | Checks related to High Integrity C++ Coding Standard.
bugprone-* | Checks that target bugprone code constructs.
cert-* | Checks related to CERT Secure Coding Guidelines.

EDIT2: With these commands we are probably fine and should be not having so many errors.

 invocation = ['clang-tidy', '-checks=cppcoreguidelines-*,clang-analyzer-*,bugprone-*,performance-*, ' +
                  'portability-*,misc-*,modernize-*,hicpp-*,cert-*,llvm-twine-local, ' + 
                  'llvm-prefer-isa-or-dyn-cast-in-conditionals, llvm-prefer-register-over-unsigned, ' + 
                  'google-readability-casting -extra-arg=-std=c++17']

The problem is just, that it doesn't find the header files and this throws mostly (afais) the errors.
So we need to tell CMake to export the compiler commands with CMAKE_EXPORT_COMPILE_COMMANDS which we can import with the flag -p for the check. I'll try that out later.

https://stackoverflow.com/a/39596822

[simonsan]

Nice work, I close this due to inactivity and continue working on it in #1157. Feel free to reopen if you add commits to this PR. Cheers.