Revert "Remove bad debug info test."

This reverts commit r201183.
The test, albeit undocumented and badly written is still testing something
useful. It will be updated in a subsequent commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201247 91177308-0d34-0410-b5e6-96231b3b80d8
Document and improve the debug-property-synth.m test based on the
information provided in the referenced radar.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201248 91177308-0d34-0410-b5e6-96231b3b80d8