IDEA: patch LLVM to make 'code_llvm_ display source location information

[maleadt]

This is a debugging patch I had floating around, which multiple people have mentioned would be very useful to have by default. It patches LLVM's AssemblyWriter to make it emit source location information:

julia> code_llvm(STDOUT, sin, Tuple{Int}, false)

define double @julia_sin_61930(i64) #0 !dbg !5 {
top:
; Filename: math.jl
; Source line: 265
  %1 = sitofp i64 %0 to double, !dbg !7
  %2 = call double @julia_sin_61931(double %1) #0, !dbg !7
  ret double %2, !dbg !7
}

Not sure if this is worth patching LLVM for, but it has helped me greatly when hunting down the origin of runtime calls in LLVM IR output. If there's interest, I could clean this up (make sure it works on all supported LLVM versions, make sure it works with @code_llvm, maybe try to still have it strip !dbg metadata, etc).

cc @andreasnoack, @jrevels

[Keno]

I'm hesitant to take any patch that's not at least submitted for review upstream. As for the change itself, it seems fine to me, though I think I'd like an option to disable the verbose line number printing.

[maleadt]

I can look into upstreaming this, but I expect that to take a while (don't have time right now, and it'll be bikeshedded to death). Some way of disabling this would be required indeed, as it breaks many FileCheck tests (not sure if we run LLVM's tests on any buildbot?).

[vtjnash]

I'd still like to see this upstreamed :). I've so far found that the LLVM folks are pretty happy to take good patches and not bikeshed them to death. Probably fine to just propose a command line argument / global configuration variable that sets the default value for an argument to the AssemblyWriter constructor.