[DebugInfo] Verify DISubprogram has a type (#194556)
Require DISubprogram metadata to carry a non-null type in the verifier.
LangRef specifies that the `type:` field of `DISubprogram` points to a
`DISubroutineType`. This patch diagnoses malformed debug info where the
field is omitted or resolves to null, while preserving the existing wrong-type
diagnostic for non-DISubroutineType operands.
Update hand-written LLVM IR tests to use valid DISubprogram metadata
where they are not intentionally testing malformed debug info. These tests now
use minimal DISubroutineType metadata so the new verifier check does not mask
their original coverage.
Fixes #186557
