[clang] [Clang] Fix null pointer dereference in enum debug info generation (PR #97105)

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smanna12 wrote:

> Thanks @smanna12. I think this looks ok; returning null here does appear to be consistent with other overloads of `CreateTypeDefinition` for `RecordType` and `ObjCInterfaceType`. I agree with @Michael137 that it would be nice to have an example that fails the added condition. The code already checks for an incomplete type so is presumably intended to handle such types. Perhaps we are missing a test though? Presumably one that uses a forward declareable enum type?

I have added test. I could not able to figure out about how we can cover the scenario where getDefinition() returns nullptr. 

https://github.com/llvm/llvm-project/pull/97105