Converted hash collision error into a fatal type error.

The check is performed during ContractLevelChecker. However, the TypeChecker throws in certain cases
when it encounters functions, whose hashes collide.

test/libsolidity/syntaxTests/nameAndTypeResolution/hash_collision_in_abstract_contract.sol

@@ -0,0 +1,13 @@
+// This contract used to throw
+abstract contract D {
+    function gsf() public {}
+    function tgeo() public {}
+}
+contract C {
+    D d;
+    function g() public returns (uint) {
+        d.d;
+    }
+}
+// ----
+// TypeError 1860: (31-113): Function signature hash collision for tgeo()