Add error for bad cast from *T to *[n]T

Casting `*T` to `*[1]T` should still work, but every other length
will now be a compiler error instead of a potential OOB access.

Author: InKryption

compile_errors/attempted_implicit_cast_from_T_to_long_array_ptr.zig

@@ -0,0 +1,18 @@
+export fn entry0(single: *u32) void {
+    _ = @as(*const [0]u32, single);
+}
+export fn entry1(single: *u32) void {
+    _ = @as(*const [1]u32, single);
+}
+export fn entry2(single: *u32) void {
+    _ = @as(*const [2]u32, single);
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:28: error: expected type '*const [0]u32', found '*u32'
+// :2:28: note: pointer type child 'u32' cannot cast into pointer type child '[0]u32'
+// :8:28: error: expected type '*const [2]u32', found '*u32'
+// :8:28: note: pointer type child 'u32' cannot cast into pointer type child '[2]u32'