[llvm] [ConstantMerge] Only merge constant w/unnamed_addr (PR #124050)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Okay, that's what I expected: pointer subtraction like that only works if both symbols are "local" (in the ELF sense).  ConstantMerge isn't aware of the pointer subtraction, so it blindly merges without checking for that.

unnamed_addr is a red herring: unnamed_addr is correlated with symbol linkage for C strings work, but it's not directly connected to the issue.  clang generates unnamed_addr variables with defualt linkage in some cases.

https://github.com/llvm/llvm-project/pull/124050