wrap llvm messages

[matthiaskrgr]

When using -Cpasses=lint, llvm prints its messages directly into stderr

   Compiling smallvec v0.6.10
   Compiling syn v1.0.5
Unusual: unreachable immediately preceded by instruction without side effects
  unreachable, !dbg !113
Unusual: unreachable immediately preceded by instruction without side effects
  unreachable, !dbg !113
Unusual: unreachable immediately preceded by instruction without side effects
  unreachable, !dbg !113
   Compiling bitflags v1.2.0

I wonder if we could wrap them so that it is easier to figure out that they are coming from llvm when parsing cargo/rustc output.

LLVM lint-pass: Unusual: unreachable...