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/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);