[Support] Simplify mapOptionalWithContext (NFC) (llvm#137529)

We can use "constexpt if" to combine the two variants of functions.

---------

Co-authored-by: Nikita Popov <[email protected]>

Author: kazutakahirata

llvm/include/llvm/Support/YAMLTraits.h

@@ -901,11 +901,12 @@ class IO {
   }
 
   template <typename T, typename Context>
-  std::enable_if_t<has_SequenceTraits<T>::value, void>
-  mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
-    // omit key/value instead of outputting empty sequence
-    if (this->canElideEmptySequence() && !(Val.begin() != Val.end()))
-      return;
+  void mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
+    if constexpr (has_SequenceTraits<T>::value) {
+      // omit key/value instead of outputting empty sequence
+      if (this->canElideEmptySequence() && Val.begin() == Val.end())
+        return;
+    }
     this->processKey(Key, Val, false, Ctx);
   }
 
@@ -916,12 +917,6 @@ class IO {
                                 /*Required=*/false, Ctx);
   }
 
-  template <typename T, typename Context>
-  std::enable_if_t<!has_SequenceTraits<T>::value, void>
-  mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
-    this->processKey(Key, Val, false, Ctx);
-  }
-
   template <typename T, typename Context, typename DefaultT>
   void mapOptionalWithContext(const char *Key, T &Val, const DefaultT &Default,
                               Context &Ctx) {