Adding tests for DebugIR pass
- lit tests verify that each line of input LLVM IR gets a !dbg node and a
  corresponding entry of metadata that contains the line number 
- unit tests verify that DebugIR works as advertised in the interface
- refactored some useful IR generation functionality from the MCJIT unit tests
  so it can be reused



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185212 91177308-0d34-0410-b5e6-96231b3b80d8