Fix MSAN failure on Function destruction

[amaiorano]

Summary

When Function is destroyed, GlobalValue base class is destroyed, then
Value destructor would call use_empty, which ultimately attempts to
downcast 'this' to GlobalValue. This is UB, and is caught my MSAN as
accessing uninitialized memory.

Call materialized_use_empty, which doesn't call
assertModuleIsMaterializedImpl().

[eugenis]

Please explain in a code comment why use_empty can not be used here.

[eugenis]

LGTM

llvm/lib/IR/Value.cpp

if (isUsedByMetadata())
     ValueAsMetadata::handleDeletion(this);
 
 #ifndef NDEBUG      // Only in -g mode...
   // Check to make sure that there are no uses of this value that are still
   // around when the value is destroyed.  If there are, then we have a dangling
   // reference and something is wrong.  This code is here to print out where
   // the value is still being referenced.
   //
-  if (!use_empty()) {
+  // Note that use_empty() cannot be called here, as it eventually downcasts
+  // 'this' to GlobalValue (derived class of Value), but GlobalValue has already
+  // been destructed, so accessing it is UB.
+  //
+  if (!materialized_use_empty()) {
     dbgs() << "While deleting: " << *VTy << " %" << getName() << "\n";
     for (auto *U : users())
       dbgs() << "Use still stuck around after Def is destroyed:" << *U << "\n";
   }
 #endif
-  assert(use_empty() && "Uses remain when a value is destroyed!");
+  assert(materialized_use_empty() && "Uses remain when a value is destroyed!");
 
   // If this value is named, destroy the name.  This should not be in a symtab
   // at this point.
   destroyValueName();
 }
 
 void Value::deleteValue() {
   switch (getValueID()) {