Qmake: More [[fallthrough]] warnings

Change-Id: I45acf2b2f6bd11a144786e076d87398d729fe2d8
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/shared/proparser/qmakebuiltins.cpp

@@ -287,7 +287,7 @@ QMakeEvaluator::quoteValue(const ProString &val)
                 break;
             case 32:
                 quote = true;
-                // fallthrough
+                [[fallthrough]];
             default:
                 ret += c;
                 break;

src/shared/proparser/qmakeevaluator.cpp

@@ -1662,7 +1662,7 @@ QMakeEvaluator::VisitReturn QMakeEvaluator::expandVariableReferences(
                 tokPtr++;
                 continue;
             }
-            // fallthrough
+            [[fallthrough]];
         default:
             Q_ASSERT_X(false, "expandVariableReferences", "Unrecognized token");
             break;
@@ -2138,7 +2138,7 @@ QString QMakeEvaluator::formatValue(const ProString &val, bool forceQuote)
                 break;
             case 32:
                 quote = true;
-                // fallthrough
+                [[fallthrough]];
             default:
                 ret += c;
                 break;

src/shared/proparser/qmakeparser.cpp

@@ -1563,7 +1563,7 @@ static bool getBlock(const ushort *tokens, int limit, int &offset, QString *outS
             case TokRemove:
             case TokReplace:
                 // The parameter is the sizehint for the output.
-                // fallthrough
+                [[fallthrough]];
             case TokLine: {
                 ushort dummy;
                 ok = getUshort(tokens, limit, offset, &dummy, outStr);