```
template-id not allowed for constructor/destructor in C++20
```

About (possibly) unused functions.

…ings

The function actually parses an unsigned integer.

```
warning: Rethrowing current exception with 'throw;', it seems there is no current
exception to rethrow. If there is no current exception this calls std::terminate().
```

Comment out unused & problematic code:
```
crengine/include/lvref.h:395:16: warning: Reference to temporary returned. [returnTempReference]
   return LVRef(NULL);
               ^
crengine/include/lvref.h:396:15: warning: Reference to temporary returned. [returnTempReference]
  return LVRef( new T( *_ptr ) );
```

```
warning: Value stored to 'pDiag' is never read [clang-analyzer-deadcode.DeadStores]
```

```
warning: Rethrowing current exception with 'throw;', it seems there is no current
exception to rethrow. If there is no current exception this calls std::terminate().
```

- fix cppcheck & clang-tidy configurations to avoid parsing, processing,
  and/or spurious errors.
- parallelize all checks to speed up linting (which is much slower now
  that cppcheck & clang-tidy can actually do their jobs).
- always lint all files (we care about), instead of trying to determine
  what changed: even if a source file, say `crengine/src/lvtinydom.cpp`,
  was not changed in a PR, some change to included headers may break
  something that could be detected by one of the checks.
- plus colors, groups and error annotations on Github Actions!

- cppcheck does not like it
- it does not match our default configuration on koreader anyway
  (where we use lunasvg)

To make cppcheck happy.

Simplify the code and make cppcheck happy.

- it's already stubbed anyway, stub it more!
- I have seen at least one crash with the default stub (clang emulator)
- cppcheck is not happy about it