[Clang][NFC] Introduce no local variable to avoid use after move (#139784)

Static analysis flagged the use of Left.size() because we just moved out
of Left and that would be undefined behavior. Fix is to take the size
and store it in a local variable instead.

Author: shafik

clang/lib/Sema/SemaConcept.cpp

@@ -1999,8 +1999,9 @@ FormulaType SubsumptionChecker::Normalize(const NormalizedConstraint &NC) {
   });
 
   if (NC.getCompoundKind() == FormulaType::Kind) {
+    auto SizeLeft = Left.size();
     Res = std::move(Left);
-    Res.reserve(Left.size() + Right.size());
+    Res.reserve(SizeLeft + Right.size());
     std::for_each(std::make_move_iterator(Right.begin()),
                   std::make_move_iterator(Right.end()), Add);
     return Res;