Add separate AstInitialStatic node for static initializers

Static variable initializers run before initial blocks, so use an
explicitly different procedure type for them. This also enables us to
now raise errors for assignments to const variables in initial blocks.

src/V3LinkParse.cpp

@@ -275,7 +275,7 @@ private:
                 if (nodep->lifetime().isAutomatic()) {
                     nodep->addNextHere(new AstInitialAutomatic{newfl, assp});
                 } else {
-                    nodep->addNextHere(new AstInitial{newfl, assp});
+                    nodep->addNextHere(new AstInitialStatic{newfl, assp});
                 }
             }  // 4. Under blocks, it's an initial value to be under an assign
             else {