Add flag to lli to enable debugging of IR when used with MCJIT.
- warn users when -debug-ir is used with old JIT engine (only partial debug
  info is available) 

For example, to debug an IR file with GDB (that supports JIT registration), do:

$ gdb --args lli -use-mcjit -debug-ir testcase.ll
(gdb) break main
(gdb) run
<Process continues to lli main>
(gdb) continue
<Process continues to testcase.ll main()
(gdb) step
<Now stepping through the LLVM IR in testcase.ll>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185197 91177308-0d34-0410-b5e6-96231b3b80d8