[Support] Fix copy-paste errors in YAMLTraits.h (llvm#157022)

has_MappingEnumInputTraits and its EmptyContext specialization define
Signature_validate to check for a member function named enumInput.
Note that in other YAML traits, Signature_foo is used to test the
presence of member method foo.

This patch fixes these copy-paste errors.

Author: kazutakahirata

llvm/include/llvm/Support/YAMLTraits.h

@@ -406,19 +406,19 @@ template <class T> struct has_MappingValidateTraits<T, EmptyContext> {
 
 // Test if MappingContextTraits<T>::enumInput() is defined on type T.
 template <class T, class Context> struct has_MappingEnumInputTraits {
-  using Signature_validate = void (*)(class IO &, T &);
+  using Signature_enumInput = void (*)(class IO &, T &);
 
-  template <class U> using check = SameType<Signature_validate, &U::enumInput>;
+  template <class U> using check = SameType<Signature_enumInput, &U::enumInput>;
 
   static constexpr bool value =
       is_detected<check, MappingContextTraits<T, Context>>::value;
 };
 
 // Test if MappingTraits<T>::enumInput() is defined on type T.
 template <class T> struct has_MappingEnumInputTraits<T, EmptyContext> {
-  using Signature_validate = void (*)(class IO &, T &);
+  using Signature_enumInput = void (*)(class IO &, T &);
 
-  template <class U> using check = SameType<Signature_validate, &U::enumInput>;
+  template <class U> using check = SameType<Signature_enumInput, &U::enumInput>;
 
   static constexpr bool value = is_detected<check, MappingTraits<T>>::value;
 };