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The definition of this struct changes in LLVM 12 due to the addition of branch coverage support. To avoid future mismatches, declare our own struct and then convert between them.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.