Fix format string in bindings/go/llvm/ir_test.go (PR40561)

The test started failing for me recently. I don't see any changes around
this code, so maybe it's my local go version that changed or something.

The error seems real to me: we're trying to print an Attribute with %d.
The test talks about "attribute masks" I'm not sure what that refers to,
but I suppose we could print the raw pointer value, since that's
what the test seems to be comparing.

Differential revision: https://reviews.llvm.org/D57672

llvm-svn: 353155