QTesselator: disable for-loop analysis on Clang

Caught by Clang 4:
error: variable 'k' is incremented both in the loop header and in the loop body [-Werror,-Wfor-loop-analysis]

But we don't want to change behavior unless we can prove that it's wrong
or that there's a way to refactor this without changing it.

Change-Id: Iecee112b0cd96bec8d975430a74ca548ee2f470f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/plugins/platforms/xcb/nativepainting/qtessellator.cpp

@@ -755,6 +755,8 @@ void QTessellatorPrivate::cancelCoincidingEdges()
 
         std::sort(tl, tl + testListSize);
 
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_CLANG("-Wfor-loop-analysis")
         for (int j = 0; j < testListSize; ++j) {
             if (tl[j].used)
                 continue;
@@ -773,6 +775,7 @@ void QTessellatorPrivate::cancelCoincidingEdges()
             }
             ++j;
         }
+QT_WARNING_POP
     }
     free(tl);
 }