Document legacy and IR behavior for call to zero initialized variable of function type.

Author: bshastry

test/libsolidity/semanticTests/functionTypes/call_to_zero_initialized_function_type_ir.sol

@@ -0,0 +1,27 @@
+contract Other {
+    function addTwo(uint256 x) public returns (uint256) {
+        return x + 2;
+    }
+}
+contract C {
+    function (function (uint) external returns (uint)) internal returns (uint) ev;
+    function (uint) external returns (uint) x;
+
+    function store(function(uint) external returns (uint) y) public {
+         x = y;
+    }
+
+    function eval(function(uint) external returns (uint) y) public returns (uint) {
+        return y(7);
+    }
+
+    function t() public returns (uint256) {
+        this.store((new Other()).addTwo);
+        // This call panics
+        return ev(x);
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// t() -> FAILURE, hex"4e487b71", 0x51

test/libsolidity/semanticTests/functionTypes/call_to_zero_initialized_function_type_legacy.sol

@@ -0,0 +1,27 @@
+contract Other {
+    function addTwo(uint256 x) public returns (uint256) {
+        return x + 2;
+    }
+}
+contract C {
+    function (function (uint) external returns (uint)) internal returns (uint) ev;
+    function (uint) external returns (uint) x;
+
+    function store(function(uint) external returns (uint) y) public {
+         x = y;
+    }
+
+    function eval(function(uint) external returns (uint) y) public returns (uint) {
+        return y(7);
+    }
+
+    function t() public returns (uint256) {
+        this.store((new Other()).addTwo);
+        // This call panics
+        return ev(x);
+    }
+}
+// ====
+// compileViaYul: false
+// ----
+// t() -> FAILURE