Remove LLVMMD{String,Node}InContext

[nnethercote]

They are both marked with FIXME comments saying they should be removed, because the corresponding LLVM functions are deprecated.

r? @nikic
cc @klensy

[klensy]

this: #130389

[nnethercote]

Some notes:

• I don't know much about the LLVM API, I just saw the FIXME comments and decided to try to remove them. I have assumed that it's ok to freely convert Value to Metadata with LLVMValueAsMetadata and vice versa with LLVMMetadataAsValue.
• Overall this makes the code a little uglier, with more of those conversion calls. A reasonable price to pay to avoid the deprecated functions?
• LLVMMDNodeInContext is always called in conjunction with LLVMSetMetadata and LLVMMetadataAsValue. I don't know if there's a nice way to factor that out.

[nnethercote]

this: #130389

Oh dear... interesting that it adds a Metadata type, I considered that but avoided it. Otherwise it looks very similar to mine, so I'll just close this.