Member predicate Impl::MethodCallExpr::getPositionalArgument

Gets the ith positional argument of this call.

Examples:

foo(42, "bar");    // `42` is argument 0 and `"bar"` is argument 1
foo.bar(42);       // `42` is argument 0
Foo::bar(foo, 42); // `42` is argument 0
x + y;             // `y` is argument 0
-x;                // no positional arguments
x[y];              // `y` is argument 0