[ADT] Assert that SmallVectorBase::grow_pod() successfully reallocates memory.

[dsanders]

Summary

If malloc/realloc fails then the SmallVector becomes unusable since begin() and
end() will return NULL. This is unlikely to occur but was the cause of recent
bugpoint test failures on my machine.

It is not clear whether not checking for malloc/realloc failure is a deliberate
decision and adding checks has the potential to impact compiler performance.
Therefore, this patch only adds the check to builds with assertions enabled for
the moment.

[dsanders]

ping

[bkramer]

Looks good. We'll probably need better error reporting for OOM some day but for now an assert is fine.

lib/Support/SmallVector.cpp

if (BeginX == FirstEl) {
     NewElts = malloc(NewCapacityInBytes);
 
     // Copy the elements over.  No need to run dtors on PODs.
     memcpy(NewElts, this->BeginX, CurSizeBytes);
   } else {
     // If this wasn't grown from the inline copy, grow the allocated space.
     NewElts = realloc(this->BeginX, NewCapacityInBytes);
   }
+  assert(NewElts && "Out of memory");
 
   this->EndX = (char*)NewElts+CurSizeBytes;
   this->BeginX = NewElts;
   this->CapacityX = (char*)this->BeginX + NewCapacityInBytes;
 }