[llvm] [LLVM][CodingStandard] Extend namespace qualifier rule to variables (PR #163588)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Note that this specific section is not about requiring declarations in headers but about using namespace qualifiers in definitions. Are you suggesting that it can be used/should be used for opaque classes like `RecordKeeperImpl` in TableGen? I just tested it out and it works, so yeah, I can extend the example to cover opaque classes that are declared in LLVM namespace in a header but defined in .cpp files.

https://github.com/llvm/llvm-project/pull/163588