This patch increments the "Debug Info Version" from 2 to 3. This is a nop if you just want to generate binaries. I verified that with and without this patch, when I run llgo -g on a Go source file, I get exactly the same binary.
The purpose of the patch is to make it possible to run the llvm-dis tool. Without the patch, it is impossible to disassemble files generated with llgo:
$ llgo -c -g -emit-llvm src/hello.go $ llvm-dis hello.o llvm-dis: warning: ignoring debug info with an invalid version (2) in hello.o