Internals: Add new VL_RESTORER flavours (#7866)

Restrict VL_RESTORER to be usable only with trivially copyable types.
Introduce VL_RESTORER_COPY and VL_RESTORER_CLEAR, which are more
efficient versions usable for non trivially copyable types.

VL_RESTORER_COPY semantically behaves the same as VL_RESTORER, but only
does one copy at initialization, and on scope exit restores via a move.

VL_RESTORER_CLEAR swaps the variable with an new one constructed via
the no-args constructor (e.g. empty collection), which does not require
any copying at any point.

Static assertions enforce picking one of the new flavours when copying
might be expensive.

src/V3Interface.cpp

@@ -43,7 +43,7 @@ class InlineIntfRefVisitor final : public VNVisitor {
     // VISITORS
     void visit(AstNetlist* nodep) override { iterateChildren(nodep->topModulep()); }
     void visit(AstCell* nodep) override {
-        VL_RESTORER(m_scope);
+        VL_RESTORER_COPY(m_scope);
         if (m_scope.empty()) {
             m_scope = nodep->name();
         } else {