b4c6e19ade
`lookup_debug_loc` finds a file, line, and column, which requires two binary searches. But this call site only needs the file. This commit replaces the call with `lookup_source_file`, which does a single binary search.
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.