Revert "Try to unbreak Win build differently after 9735198"

Breaks the build and failed pre-merge checks:
https://buildkite.com/llvm-project/premerge-checks/builds/54930#07373971-3d37-49cf-9def-22c0d724ee23

> llvm-project/lld/wasm/Writer.cpp:521:16: error: non-const lvalue reference to
>  type 'llvm::StringRef' cannot bind to a temporary of type 'llvm::StringRef'
>    for (auto &feature : used.keys()) {

This reverts commit 5881dcf.

Author: GMNGeoffrey

clang/lib/Driver/ToolChains/Arch/X86.cpp

@@ -59,8 +59,9 @@ std::string x86::getX86TargetCPU(const Driver &D, const ArgList &Args,
     }
     StringRef CPU = ArchMap.lookup(A->getValue());
     if (CPU.empty()) {
-      std::vector<StringRef> ValidArchs{ArchMap.keys().begin(),
-                                        ArchMap.keys().end()};
+      std::vector<StringRef> ValidArchs;
+      for (StringRef Key : ArchMap.keys())
+        ValidArchs.push_back(Key);
       sort(ValidArchs);
       D.Diag(diag::warn_drv_invalid_arch_name_with_suggestion)
           << A->getValue() << (Triple.getArch() == llvm::Triple::x86)

llvm/include/llvm/ADT/StringMap.h

@@ -478,9 +478,13 @@ class StringMapKeyIterator
   explicit StringMapKeyIterator(StringMapConstIterator<ValueTy> Iter)
       : base(std::move(Iter)) {}
 
-  StringRef operator*() const {
-    return this->wrapped()->getKey();
+  StringRef &operator*() {
+    Key = this->wrapped()->getKey();
+    return Key;
   }
+
+private:
+  StringRef Key;
 };
 
 } // end namespace llvm

llvm/unittests/ADT/StringMapTest.cpp

@@ -308,21 +308,7 @@ TEST_F(StringMapTest, InsertOrAssignTest) {
   EXPECT_EQ(0, try1.first->second.copy);
 }
 
-TEST_F(StringMapTest, IterMapKeysVector) {
-  StringMap<int> Map;
-  Map["A"] = 1;
-  Map["B"] = 2;
-  Map["C"] = 3;
-  Map["D"] = 3;
-
-  std::vector<StringRef> Keys{Map.keys().begin(), Map.keys().end()};
-  llvm::sort(Keys);
-
-  std::vector<StringRef> Expected{{"A", "B", "C", "D"}};
-  EXPECT_EQ(Expected, Keys);
-}
-
-TEST_F(StringMapTest, IterMapKeysSmallVector) {
+TEST_F(StringMapTest, IterMapKeys) {
   StringMap<int> Map;
   Map["A"] = 1;
   Map["B"] = 2;