[RemoveDIs][DebugInfo] Explicitly convert iterator to pointer for std::distance

This is a small patch attempting to fix an error with a prior patch which
caused buildbot failures on certain targets; for example:
https://lab.llvm.org/buildbot/#/builders/127/builds/60931

The compile error comes from std::distance not having an overload for a
std::array iterator and a raw pointer; this patch converts the iterator
to a pointer to resolve this.

Fixes d499df0

Author: SLTozer

llvm/lib/IR/Metadata.cpp

@@ -157,7 +157,7 @@ void DebugValueUser::handleChangedValue(void *Old, Metadata *New) {
   // NOTE: We could inform the "owner" that a value has changed through
   // getOwner, if needed.
   auto OldMD = static_cast<Metadata **>(Old);
-  ptrdiff_t Idx = std::distance(DebugValues.begin(), OldMD);
+  ptrdiff_t Idx = std::distance(&*DebugValues.begin(), OldMD);
   resetDebugValue(Idx, New);
 }