When using the vim syntax for mir, an error occurs in nvim  when opening
multiple .mir buffers. delcommand HiLink in the mir syntax file to avoid
the issue.

To reproduce:
Open an .mir file, for example
llvm/test/Codegen/X86/expand-post-ra-pseudo.mir
Open another mir file from within nvim, for example peephole.mir
```
Error detected while processing function 335[30]..<SNR>43_callback[25]..function 335[30]..<SNR>43_callback:                                                                                                                      
line   23:                                                                                                                                                                                                                       
Vim(command):E174: Command already exists: add ! to replace it: HiLink hi def link <args>   
```

This initial definition handles the yaml container and the embedding of
the inner IRs. As a stopgap, this reuses the LLVM IR syntax highlighting
for the MIR function bodies--even though it's not technically correct,
it produces decent highlighting for a first pass.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D95552