Contract level checker: Disallow free function redefinition and alias

shadowing another free function

Co-authored-by: chriseth <chris@ethereum.org>

test/libsolidity/syntaxTests/multiSource/free_function_resolution_override_virtual.sol

@@ -0,0 +1,19 @@
+==== Source: s1.sol ====
+function f() pure returns (uint) { return 1337; }
+contract C {
+  function f() public pure virtual returns (uint) {
+    return f();
+  }
+}
+==== Source: s2.sol ====
+import "s1.sol";
+function f() pure returns (uint) { return 42; }
+contract D is C {
+  function f() public pure override returns (uint) {
+    return f();
+  }
+}
+// ----
+// Warning 2519: (s1.sol:65-134): This declaration shadows an existing declaration.
+// Warning 2519: (s2.sol:85-155): This declaration shadows an existing declaration.
+// DeclarationError 1686: (s2.sol:17-64): Function with same name and parameter types defined twice.