[llvm] [DebugInfo] Verify DISubprogram has a type (PR #194556)

[Michael Buch via llvm-commits]

https://github.com/Michael137 commented:

For context, the discussions were:
* https://github.com/llvm/llvm-project/pull/193420
* https://github.com/llvm/llvm-project/pull/184299#discussion_r2877180110

If we're making the verifier stricter about this field, we should, in a separate patch, also update `BitcodeWriter`/`MetadataLoader`/`LLParser` to make this field required.

And revert `4262045ba98d11c930fec0fe02584b072ace19bd` since that null-check would then be redundant.

https://github.com/llvm/llvm-project/pull/194556