validation: make CScriptCheck and prevector swap member functions noexcept

Reason:
A swap must not fail; when a class has a swap member function, it should be declared noexcept.
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c84-a-swap-function-must-not-fail

src/prevector.h

@@ -458,7 +458,8 @@ public:
         return *item_ptr(size() - 1);
     }
 
-    void swap(prevector<N, T, Size, Diff>& other) {
+    void swap(prevector<N, T, Size, Diff>& other) noexcept
+    {
         std::swap(_union, other._union);
         std::swap(_size, other._size);
     }

src/validation.h

@@ -328,7 +328,8 @@ public:
 
     bool operator()();
 
-    void swap(CScriptCheck &check) {
+    void swap(CScriptCheck& check) noexcept
+    {
         std::swap(ptxTo, check.ptxTo);
         std::swap(m_tx_out, check.m_tx_out);
         std::swap(nIn, check.nIn);