C++: Ensure we use lvalue reference types for structured bindings

This also adds a test for rvalue reference uses in the tuple
structured binding case.

Author: jketema

cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/internal/TranslatedExpr.qll

@@ -903,7 +903,7 @@ class TranslatedStructuredBindingVariableAccess extends TranslatedNonConstantExp
   }
 
   private Type getReferenceType() {
-    result.(ReferenceType).getBaseType() = expr.getUnderlyingType()
+    result.(LValueReferenceType).getBaseType() = expr.getUnderlyingType()
   }
 
   override Instruction getInstructionRegisterOperand(InstructionTag tag, OperandTag operandTag) {

cpp/ql/lib/semmle/code/cpp/ir/internal/IRUtilities.qll

@@ -41,7 +41,7 @@ Type getVariableType(Variable v) {
         not exists(v.getInitializer()) and result = v.getType()
       else
         if isNonReferenceStructuredBinding(v)
-        then exists(ReferenceType r | r.getBaseType() = v.getUnderlyingType() | result = r)
+        then exists(LValueReferenceType r | r.getBaseType() = v.getUnderlyingType() | result = r)
         else result = v.getType()
   )
 }