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

[Rahul Joshi via cfe-commits]

jurahul wrote:

I see that the warning is not yet implemented in clang, but its in GCC, so only GCC based builds flagged it as an error. So what we really need it a GCC based build, which is not a tweak.

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