[clang] [clang-repl] Emit const variables only once (PR #65257)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

The original patch worked for `clang-repl` but results in strong linkage which I found to cause problems with modules downstream in ROOT. Instead the latest push moves the special case one level higher to `basicGVALinkageForVariable` and returns `GVA_DiscardableODR` which fixes that problem as well.

https://github.com/llvm/llvm-project/pull/65257