[clang] [clang][Modules] Keep existing global definition on type mismatch in GetOrCreateLLVMGlobal (PR #223877)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

At the AST level, all declarations of a given variable should be linked together, so CodeGen will treat them as the same global.  This should happen even if a variable has multiple definitions due to module importing.

https://github.com/llvm/llvm-project/pull/223877