Add setimmutable and loadimmutable to dialect.

test/libsolidity/syntaxTests/inlineAssembly/immutables.sol

@@ -0,0 +1,11 @@
+contract C {
+  function f() public pure {
+    assembly {
+      setimmutable("abc", 0)
+      loadimmutable("abc")
+    }
+  }
+}
+// ----
+// DeclarationError: (63-75): Function not found.
+// DeclarationError: (92-105): Function not found.