Syntax highlighting for MIR

Hello,

I’ve been writing MIR test cases and wanted some syntax highlighting, so I started work on a vim syntax.
I’m having trouble getting traction from reviewers and thought I would ask here.

• Adding a base vim syntax that reuses LLVM IR highlighting https://reviews.llvm.org/D95552
• Add some syntax that isn’t covered by LLVM IR https://reviews.llvm.org/D95553

If there’s more interest in this syntax highlighting I could make it more precise, but this already makes it much easier to read and write those test cases.

— Cassie