Add a helpful assert if LineNo would clobber ArgNo

[loladiro]

Summary

Otherwise passing in an invalid line number on accident (e.g. -1)
would cause a hard-to-decipher assert later on. Better to catch this early.

[loladiro]

I feel slightly silly for bumping this, since it's such a trivial change, but I was looking through my current patch set locally and realized this wasn't in yet.

[echristo]

I like the idea of degrading since that it's limited to 24 bits is a limitation on our part... that patch is fine to just go in though.

lib/IR/DIBuilder.cpp

   DIDescriptor Context(getNonCompileUnitScope(Scope));
   assert((!Context || Context.isScope()) &&
          "createLocalVariable should be called with a valid Context");
+  assert(((LineNo & (0xFF << 24)) == 0) &&
+         "LineNo exceeds maximum line number");
   Value *Elts[] = {HeaderBuilder::get(Tag)
                        .concat(Name)
                        .concat(LineNo | (ArgNo << 24))