[llgo] Update to use the latest IR attribute bindings

A recent commit (r286087) to the LLVM Go bindings that
changed things over to use the new attribute API broke
llgo.  This commit updates llgo accordingly.

llvm-svn: 288769
GitOrigin-RevId: 21e53508b63c3192d293dff806b26771d100f214
7 files changed