[ADT] Use adl_being/end in `hasSingleElement

This is to make sure that ADT helpers consistently use argument
dependent lookup when dealing with input ranges.

This was a part of #87936 but reverted due to buildbot failures. Now
that I have a threadripper system, I'm landing this piece-by-piece.

Author: kuhar

llvm/include/llvm/ADT/STLExtras.h

@@ -320,7 +320,7 @@ template <typename T> class Callable<T, true> {
 
 /// Returns true if the given container only contains a single element.
 template <typename ContainerTy> bool hasSingleElement(ContainerTy &&C) {
-  auto B = std::begin(C), E = std::end(C);
+  auto B = adl_begin(C), E = adl_end(C);
   return B != E && std::next(B) == E;
 }
 

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -934,10 +934,19 @@ TEST(STLExtrasTest, hasSingleElement) {
   const std::vector<int> V0 = {}, V1 = {1}, V2 = {1, 2};
   const std::vector<int> V10(10);
 
-  EXPECT_EQ(hasSingleElement(V0), false);
-  EXPECT_EQ(hasSingleElement(V1), true);
-  EXPECT_EQ(hasSingleElement(V2), false);
-  EXPECT_EQ(hasSingleElement(V10), false);
+  EXPECT_FALSE(hasSingleElement(V0));
+  EXPECT_TRUE(hasSingleElement(V1));
+  EXPECT_FALSE(hasSingleElement(V2));
+  EXPECT_FALSE(hasSingleElement(V10));
+
+  // Make sure that we use the `begin`/`end` functions
+  // from `some_namespace`, using ADL.
+  some_namespace::some_struct S;
+  EXPECT_FALSE(hasSingleElement(S));
+  S.data = V1;
+  EXPECT_TRUE(hasSingleElement(S));
+  S.data = V2;
+  EXPECT_FALSE(hasSingleElement(S));
 }
 
 TEST(STLExtrasTest, hasNItems) {