[clang] [NFC][clang][TableGen] Remove redundant llvm:: namespace qualifier (PR #108627)

[Jan Patrick Lehr via cfe-commits]

jplehr wrote:

Hi, we see the same errors on our buildbots, two as examples:

https://lab.llvm.org/buildbot/#/builders/30/builds/6250
https://lab.llvm.org/buildbot/#/builders/140/builds/6648

https://github.com/llvm/llvm-project/pull/108627