IRGen: Fix a use-after-free error in ShadowStackSlots creation

The `ShadowStackSlots` map uses a `StringRef` to the `SmallString<4>` entry in
the `AnonymousVariables` map. When `AnonymousVariables` is resized the pointer
to the inline string entry will no longer be valid.

No reproducer.

rdar://102982240

Author: aschwaighofer

lib/IRGen/IRGenSIL.cpp

@@ -403,7 +403,7 @@ class IRGenSILFunction :
   /// Keeps track of the mapping of source variables to -O0 shadow copy allocas.
   llvm::SmallDenseMap<StackSlotKey, Address, 8> ShadowStackSlots;
   llvm::SmallDenseMap<llvm::Value *, Address, 8> TaskAllocStackSlots;
-  llvm::SmallDenseMap<Decl *, SmallString<4>, 8> AnonymousVariables;
+  llvm::SmallDenseMap<Decl *, Identifier, 8> AnonymousVariables;
   /// To avoid inserting elements into ValueDomPoints twice.
   llvm::SmallDenseSet<llvm::Value *, 8> ValueVariables;
   /// Holds the DominancePoint of values that are storage for a source variable.
@@ -653,15 +653,17 @@ class IRGenSILFunction :
   }
 
   StringRef getOrCreateAnonymousVarName(VarDecl *Decl) {
-    llvm::SmallString<4> &Name = AnonymousVariables[Decl];
+    Identifier &Name = AnonymousVariables[Decl];
     if (Name.empty()) {
       {
-        llvm::raw_svector_ostream S(Name);
+        llvm::SmallString<64> NameBuffer;
+        llvm::raw_svector_ostream S(NameBuffer);
         S << "$_" << NumAnonVars++;
+        Name = IGM.Context.getIdentifier(NameBuffer);
       }
       AnonymousVariables.insert({Decl, Name});
     }
-    return Name;
+    return Name.str();
   }
 
   template <class DebugVarCarryingInst>