[clang] Use StringRef::{starts,ends}_with (NFC) (#75149)

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

Author: kazutakahirata

clang/include/clang/Basic/Attr.td

@@ -2878,7 +2878,7 @@ def Target : InheritableAttr {
 
       for (auto &Feature : AttrFeatures) {
         Feature = Feature.trim();
-        if (Feature.startswith("arch="))
+        if (Feature.starts_with("arch="))
           return Feature.drop_front(sizeof("arch=") - 1);
       }
       return "";
@@ -2896,8 +2896,8 @@ def Target : InheritableAttr {
       for (auto &Feature : AttrFeatures) {
         Feature = Feature.trim();
 
-        if (!Feature.startswith("no-") && !Feature.startswith("arch=") &&
-            !Feature.startswith("fpmath=") && !Feature.startswith("tune="))
+        if (!Feature.starts_with("no-") && !Feature.starts_with("arch=") &&
+            !Feature.starts_with("fpmath=") && !Feature.starts_with("tune="))
           Out.push_back(Feature);
       }
     }

clang/include/clang/Basic/IdentifierTable.h

@@ -511,7 +511,7 @@ class alignas(IdentifierInfoAlignment) IdentifierInfo {
   ///   function(<#int x#>);
   /// \endcode
   bool isEditorPlaceholder() const {
-    return getName().startswith("<#") && getName().endswith("#>");
+    return getName().starts_with("<#") && getName().ends_with("#>");
   }
 
   /// Determine whether \p this is a name reserved for the implementation (C99