[DebugInfo] Enforce DISubprogram type as required in LLParser (#196299)
Add a parse-time check so that LLParser treats the `type:` field of
`DISubprogram` as required and non-null. Attempting to assemble IR with
a missing or null `type:` now produces a clear parse-time error
("missing required field 'type'" or "'type' cannot be null") rather
than relying solely on the verifier warning added in #194556.
Update existing hand-written LLVM IR tests to supply a minimal valid
`DISubroutineType` where they are not intentionally testing malformed
debug info.
**Scope.** This patch covers the LLParser path only. Two entry points
are intentionally deferred:
- *MetadataLoader*: pre-LLVM-5 bitcode legitimately stores null
`DISubprogram.type`; auto-upgrading that field requires more careful
backward-compatibility handling (see
`llvm/test/Bitcode/dityperefs-3.8.ll`).
- *BitcodeWriter*: IR written to bitcode has already passed the
verifier, which guarantees a non-null type; no additional check is
needed in the writer.
Follow-up to #194556.
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