[PATCH] Add a new pass to compute DWARF path discriminators.

[Diego Novillo]

Hi echristo,

DWARF discriminators are used to distinguish multiple control flow paths
on the same source location. When this happens, instructions across
basic block boundaries will share the same debug location.

This pass detects this situation and adds a new discriminator value to
one of the two instructions so that their debug location has a different
discriminator value.

Discriminators are associated with the lexical scope of the instruction.
This is then emitted to the object file in DwarfDebug::recordSourceLine.

NOTE: This still does not work fully. It ICEs the compiler in certain
      cases and I don't think it's emitting discriminators everywhere
      I need them. I mostly want to know if I'm on the right path.

http://llvm-reviews.chandlerc.com/D2691

Files:
  include/llvm/DebugInfo.h
  include/llvm/InitializePasses.h
  include/llvm/Transforms/Scalar.h
  lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  lib/IR/DebugInfo.cpp
  lib/IR/LLVMContextImpl.h
  lib/MC/MCObjectStreamer.cpp
  lib/Transforms/Scalar/SampleProfile.cpp
  lib/Transforms/Utils/AddDiscriminators.cpp
  lib/Transforms/Utils/CMakeLists.txt
  lib/Transforms/Utils/Utils.cpp
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