[llvm] [AMDGPU][LowerModuleLDS] Avoid crash on zero-sized types (PR #84742)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The issue reporter said that this was a coding error, their issue wasn't about it not being handled but about the fact that the compiler crashed in such cases. Reporting an error here is good enough. It should ideally be reported in Clang but I'd need help from a more knowledgeable contributor to understand why Clang doesn't report this yet and where the fix should be

If we're going to make this an error, it should be a proper diagnostic in the context instead of report_fatal_error. But we already handle this situation in the external case, so I don't see the difficulty in just extending that to ignore the linkage, which shouldn't be a factor 

https://github.com/llvm/llvm-project/pull/84742