SIL: Fix assertion failure after deleting instructions that contain unresolved local archetypes

We maintained a counter of the number of pending local archetypes
that had not yet been defined. However, if an instruction that
references a pending local archetype was deleted before the
local archetype was defined, the counter would never decrement.

Before reading the counter value, garbage collect any inserted
placeholders that have no uses. These correspond to pending
local archetypes that are no longer in use and will never be
defined.

Author: slavapestov

lib/SIL/IR/SILModule.cpp

@@ -688,6 +688,23 @@ SILValue SILModule::getRootLocalArchetypeDef(CanLocalArchetypeType archetype,
 }
 
 bool SILModule::hasUnresolvedLocalArchetypeDefinitions() {
+  // Garbage collect dead placeholders first.
+  llvm::DenseMap<LocalArchetypeKey, SILValue> newLocalArchetypeDefs;
+
+  for (auto pair : RootLocalArchetypeDefs) {
+    if (auto *placeholder = dyn_cast<PlaceholderValue>(pair.second)) {
+      if (placeholder->use_empty()) {
+        --numUnresolvedLocalArchetypes;
+        ::delete placeholder;
+        continue;
+      }
+    }
+
+    newLocalArchetypeDefs.insert(pair);
+  }
+
+  std::swap(newLocalArchetypeDefs, RootLocalArchetypeDefs);
+
   return numUnresolvedLocalArchetypes != 0;
 }
 
@@ -753,6 +770,8 @@ void SILModule::notifyAddedInstruction(SILInstruction *inst) {
       auto *placeholder = cast<PlaceholderValue>(val);
       placeholder->replaceAllUsesWith(dependency);
       ::delete placeholder;
+
+      assert(numUnresolvedLocalArchetypes > 0);
       numUnresolvedLocalArchetypes--;
     }
     val = dependency;