ControlFlowAnalyser: Also consider called functions in a flow

test/libsolidity/syntaxTests/multiSource/libraries_control_flow_analysis.sol

@@ -0,0 +1,16 @@
+==== Source: s1.sol ====
+library L
+{
+	function normal() public pure returns (uint) { return 1337; }
+	function reverting() public pure returns (uint) { revert(); }
+}
+==== Source: s2.sol ====
+import "s1.sol";
+contract C
+{
+	function foo() public pure returns (uint) { L.normal(); }
+	function bar() public pure returns (uint) { L.reverting(); }
+}
+// ----
+// Warning 6321: (s2.sol:67-71): Unnamed return variable can remain unassigned. Add an explicit return with value to all non-reverting code paths or name the variable.
+// Warning 6321: (s2.sol:126-130): Unnamed return variable can remain unassigned. Add an explicit return with value to all non-reverting code paths or name the variable.