[DebugInfo] don't make an MDTuple just to leak it

There does not seem to be any purpose to allocating this object, which
is Metadata, so LLVM will simply leak it until the context is destroyed
(the subprogram metadata it contains are distinct, so there is little
chance of it being reused later). This should not have a functional
change.

This seems to be left over from an earlier design, when it used to call
```
CUNode->replaceSubprograms(SPs.get());
```
here also. (removed in http://reviews.llvm.org/D19034)

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D147469