[5.9] [Sema] Add shouldWalkIntoTapExpression override for SingleValueStmtUsageChecker

This matches all the other MiscDiagnostics walkers,
and is needed to avoid emitting a duplicate diagnostic
for a string interpolation, since on 5.9 we
type-check TapExprs separately. This isn't needed
on main/5.10 since we now type-check them together
with the whole expression.

Author: hamishknight

lib/Sema/MiscDiagnostics.cpp

@@ -3894,6 +3894,8 @@ class SingleValueStmtUsageChecker final : public ASTWalker {
     return MacroWalking::ArgumentsAndExpansion;
   }
 
+  bool shouldWalkIntoTapExpression() override { return false; }
+
   AssignExpr *findAssignment(Expr *E) const {
     // Don't consider assignments if we have a parent expression (as otherwise
     // this would be effectively allowing it in an arbitrary expression