ASTContext: Recompute the insert position in getSpecializedConformance after the SpecializedProtocolConformance constructor

The constructor can modify the SpecializedConformances FoldingSet so that the
insertPos is no longer valid. It calls computeConditionalRequirements which
calls Type::subst which recursively can call getSpecializedConformance again.

This bug manifest itself either as memory error with libgmalloc or as
spurious errors later on.

rdar://42082352

Author: aschwaighofer

lib/AST/ASTContext.cpp

@@ -1835,6 +1835,8 @@ ASTContext::getSpecializedConformance(Type type,
   auto result
     = new (*this, arena) SpecializedProtocolConformance(type, generic,
                                                         substitutions);
+  auto node = specializedConformances.FindNodeOrInsertPos(id, insertPos);
+  assert(!node);
   specializedConformances.InsertNode(result, insertPos);
   return result;
 }