test_values::test_metadata triggers assertion in llvm 10

[nlewycky]

To Reproduce

$ cargo test --features=llvm10-0 test_values::test_metadata
all-0b042a69785d4445: ../lib/IR/Core.cpp:876: llvm::MDNode* extractMDNode(llvm::MetadataAsValue*): Assertion `(isa<MDNode>(MD) || isa<ConstantAsMetadata>(MD)) && "Expected a metadata node or a canonicalized constant"' failed.

Describe the Bug
The problem is on this line:

module.add_global_metadata("my_md", &md_string)

We can only call LLVMAddNamedMetadataOperand on an MDNode or a "ConstantAsMetadata" which is ValueAsMetadata for llvm::Constant.

Expected Behavior
The test is trying to build the module:

!my_md = !{!"lots of metadata here", !0}
!0 = !{i1 false, float 0.000000e+00}

which is not a valid module in LLVM 10, nor in 9 or 8 which is all I tested. I'm pretty sure it used to be at some point so we may need to figure out when that changed and update the Module::add_global_metadata to either build an MDNode wrapping an MDString when that was made invalid, or if this was never valid then to change its type signature to not accept MDStrings.

LLVM Version (please complete the following information):

• LLVM Version: 10
• Inkwell Branch Used: master

Additional Context
The transform I'm proposing that we create:

!my_md = !{!0, !1}
!0 = !{!"lots of metadata here"}
!1 = !{i1 false, float 0.000000e+00}

when asked to add a string directly to a global metadata.

I'm not sure about the ConstantAsMetadata thing, I tried putting constants into metadata nods and it was not accepted by the parser.