[llvm] [BOLT] Modify MCPlus annotation internals. NFCI. (PR #70412)

[via llvm-commits]

maksfb wrote:

> One more remark: I think `MCPlusBuilder::setLabel` should also be made `const` here.

Thanks. I'll add `const` it in #70147. 

https://github.com/llvm/llvm-project/pull/70412