Use yul::AstWalker to resolve assembly symbols

test/libsolidity/syntaxTests/inlineAssembly/invalid/nested_function_local_access.sol

@@ -0,0 +1,12 @@
+contract C {
+  function f() public pure returns (uint x) {
+    assembly {
+      function f1() {
+        function f2() { }
+        x := 2
+      }
+    }
+  }
+}
+// ----
+// DeclarationError: (130-131): Cannot access local Solidity variables from inside an inline assembly function.