Warn on unused return values in guidance code, resolves #2686.

https://github.com/Project-OSRM/osrm-backend/pull/2685/files

fixes an issue where we did

    elongate(fstStep, sndStep);

instead of

    newStep = elongate(fstStep, sndStep);

we didn't get any warnings.

The only way to trigger a warning here is to use

```cpp
__attribute__((warn_unused_result))
```

This changeset does exactly that: for the new guidance code prone to
these kind of issue we add such an attribute to the declaration.

include/util/guidance/toolkit.hpp

@@ -6,6 +6,7 @@
 #include "extractor/guidance/turn_instruction.hpp"
 #include "engine/guidance/route_step.hpp"
 #include "engine/phantom_node.hpp"
+#include "util/attributes.hpp"
 #include "util/guidance/bearing_class.hpp"
 #include "util/guidance/entry_class.hpp"
 #include "util/simple_logger.hpp"
@@ -50,6 +51,7 @@ inline extractor::guidance::DirectionModifier::Enum getTurnDirection(const doubl
 }
 
 // swaps left <-> right modifier types
+OSRM_ATTR_WARN_UNUSED
 inline extractor::guidance::DirectionModifier::Enum
 mirrorDirectionModifier(const extractor::guidance::DirectionModifier::Enum modifier)
 {