[clang] Merge similar Clang Thread Safety attributes (PR #135561)

[Aaron Puchert via cfe-commits]

aaronpuchert wrote:

I think it's obscure enough to not need to be mentioned. Since I went with the less intrusive variant of downgrading an error to warning, it should not break anyone's code.

I'd be open to add `ErrorDiag` on all thread safety attributes, but I'd do so in a separate change in case we break some user code and it has to be reverted.

https://github.com/llvm/llvm-project/pull/135561