[ConstantFold] Make areGlobalsPotentiallyEqual less aggressive.

[efriedma]

Summary

In particular, we shouldn't make assumptions about globals which are unnamed_addr: we can fold them together with other globals.

Also while I'm here, use isInterposable() instead of trying to explicitly name all the different kinds of weak linkage.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47090

[fhahn]

LGTM, thanks!

llvm/test/Assembler/ConstantExprNoFold.ll

47

Looks like the precommit checks complain about this. Shouldn't this be c"...." for string constants? Also, it seems like escape sequences are not supported by the bitcode parser and \0 counts as 2 chars.