[ADT] Fix a warning

This patch fixes:

  llvm/unittests/ADT/DenseMapTest.cpp:94:25: error: unused function
  'getTestValue' [-Werror,-Wunused-function]

Author: kazutakahirata

llvm/unittests/ADT/DenseMapTest.cpp

@@ -91,10 +91,6 @@ std::optional<uint32_t> getTestKey(int i, std::optional<uint32_t> *) {
   return i;
 }
 
-std::optional<uint32_t> getTestValue(int i, std::optional<uint32_t> *) {
-  return 42 + i;
-}
-
 // Test fixture, with helper functions implemented by forwarding to global
 // function overloads selected by component types of the type parameter. This
 // allows all of the map implementations to be tested with shared