[clang] [LifetimeSafety] Replace `getCrossTUDecl` with `getCanonicalDeclForAttr` (PR #198784)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

If we still want to enforce that the declaration in the header needs the annotation, we still need `getCrossTUDecl`, so we cannot get rid of it. 

https://github.com/llvm/llvm-project/pull/198784