Do assignment lvalue error checking in type checking

Added some tests related to this.
Also improved source file position reporting in error reporting.

tests_errors/lvalue-1.ispc

@@ -0,0 +1,8 @@
+// Left hand side of assignment statement can't be assigned to
+
+int foo() {return 2;}
+
+int bar()
+{
+    foo() = 2;
+}