qmake: use [[fallthrough]]

Since qt is on C++17, it's possible too use the [[fallthrough]]
attribute instead of a // Fallthrough comment, that may be stripped by
compiler launchers (ccache/icecc), causing -Wimplicit-fallthrough
warnings.

Change-Id: Ic809cf4e95b2160fac591b3802fa123e705337cc
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 84537e6dc22030004c2aec7285641e721da627cf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

qmake/generators/win32/msvc_objectmodel.cpp

@@ -862,7 +862,7 @@ bool VCCLCompilerTool::parseOption(const char* option)
                 }
                 break;
             }
-            // Fallthrough
+            [[fallthrough]];
         default:
             found = false; break;
         }