Try to unbreak Win build differently after 973519826e

Looks like the MS STL wants StringMapKeyIterator::operator*() to be const.
Return the result by copy instead of reference to do that.
Assigning to a hash map key iterator doesn't make sense anyways.

Also reverts 123f811fe5 which is now hopefully no longer needed.

Differential Revision: https://reviews.llvm.org/D109167

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

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

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

llvm/unittests/ADT/StringMapTest.cpp

@@ -308,7 +308,21 @@ TEST_F(StringMapTest, InsertOrAssignTest) {
   EXPECT_EQ(0, try1.first->second.copy);
 }
 
-TEST_F(StringMapTest, IterMapKeys) {
+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) {
   StringMap<int> Map;
   Map["A"] = 1;
   Map["B"] = 2;