[ms-inline asm] Don't diagnose an empty lookup for inline assmebly.  This happen
for labels in inline assembly that aren't in the lookup tables.  E.g.,

  __asm {
   a:
   jmp a
  }

rdar://13983623


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182659 91177308-0d34-0410-b5e6-96231b3b80d8