[clang codegen][regression] Add dso_local/hidden/etc. markings to VTT...

[clang codegen][regression] Add dso_local/hidden/etc. markings to VTT definitions and declarations (#72452)

https://reviews.llvm.org/D128482 regressed certain cases of VTT emission
which are no longer hidden with -fvisibility=hidden.

Fix this regression by marking both declarations and definitions.

Fixes [clang codegen][regression] VTT definitions missing
dso_local/hidden/etc markings #72451