DebugInfo: Add source code/build instructions for split-dwarf-dwp symbolizer test

Addressing post-commit code review feedback from Paul Robinson on
r303609.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304080 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed