ir: add metadata read APIs

[luoliwoshang]

Summary

• add receiver-style APIs to read named metadata and MDNode operands from LLVM IR
• add MDString helpers on Value for metadata parsing workflows
• add a round-trip test that emits llgo-style metadata, verifies printed IR, parses it back, and reads it through the new APIs

Testing

• go test ./...
• go test -run TestNamedMetadataRoundTrip ./...

[gemini-code-assist]

Code Review

This pull request introduces several methods to the Module and Value types to support the retrieval of named metadata and metadata node operands, as well as the identification and extraction of MDStrings. A comprehensive test case has been added to verify these operations. Feedback was provided regarding a redundant CString allocation in the NamedMetadataOperands function, which could be optimized to improve efficiency.

[fennoai]

Code Review: ir: add metadata read APIs

Good addition of metadata read APIs. A few issues worth addressing, ranging from a safety concern to test robustness.