[clang] Remove unused declaration IgnoreLinkageSpecDecls

The declaration and its corresponding function definition were removed
once by:

  commit 02093906
  Author: Nathan Sidwell <nathan@acm.org>
  Date:   Mon Feb 14 10:19:04 2022 -0800

However, the declaration was added back without a corresponding
function definition a few days later by:

  commit 18ead233
  Author: Peter Collingbourne <peter@pcc.me.uk>
  Date:   Thu Feb 17 11:23:33 2022 -0800

This is most likely a rebasing error.