Change DIBuilderCreateEnumerator signature to match LLVM 9

[tmiasko]

• Change DIBuilderCreateEnumerator signature to match LLVM 9 C API.
• Use provided is unsigned flag when emitting enumerators.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ecstatic-morse (or someone else) soon.

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[ecstatic-morse]

Some questions about ownership need to be resolved before this is merged. Also left a nit about using as casts for discriminants.

[ecstatic-morse]

Thanks @tmiasko! LGTM, although I'm gonna defer to someone who works on this code, since I don't quite understand why this worked before if we were using the wrong signature.

r? @nagisa

[tmiasko]

Oh sorry, I didn't made it clear in PR description. The PR prepares for migration
to the C API that is available since LLVM 9, without actually mgirating it yet, sice
we support older versions of LLVM.

Thus the code continues to use own wrapper LLVMRustDIBuilderCreateEnumerator rather than
official one LLVMDIBuilderCreateEnumerator, but now they have exactly the same type
signature and arguments.

[ecstatic-morse]

Ah, I see now.

r? @ecstatic-morse
@bors r+ rollup

[bors]

📌 Commit 30650f8 has been approved by ecstatic-morse

[ecstatic-morse]

Note to self: llvm::StringRef explicitly does not require the string to be null-terminated. It does, however, have a const char* constructor for null-terminated strings, which is what we were using prior to this PR.