[ADT] Add StringRef::{starts,ends}_with(char) (#90311)

This patch adds to StringRef the equivalent of
std::string_view::{starts,ends}_with(char) in C++20.

Author: kazutakahirata

llvm/include/llvm/ADT/StringRef.h

@@ -258,6 +258,9 @@ namespace llvm {
       return Length >= Prefix.Length &&
              compareMemory(Data, Prefix.Data, Prefix.Length) == 0;
     }
+    [[nodiscard]] bool starts_with(char Prefix) const {
+      return !empty() && front() == Prefix;
+    }
 
     /// Check if this string starts with the given \p Prefix, ignoring case.
     [[nodiscard]] bool starts_with_insensitive(StringRef Prefix) const;
@@ -268,6 +271,9 @@ namespace llvm {
              compareMemory(end() - Suffix.Length, Suffix.Data, Suffix.Length) ==
                  0;
     }
+    [[nodiscard]] bool ends_with(char Suffix) const {
+      return !empty() && back() == Suffix;
+    }
 
     /// Check if this string ends with the given \p Suffix, ignoring case.
     [[nodiscard]] bool ends_with_insensitive(StringRef Suffix) const;

llvm/unittests/ADT/StringRefTest.cpp

@@ -368,6 +368,8 @@ TEST(StringRefTest, StartsWith) {
   EXPECT_TRUE(Str.starts_with("he"));
   EXPECT_FALSE(Str.starts_with("helloworld"));
   EXPECT_FALSE(Str.starts_with("hi"));
+  EXPECT_TRUE(Str.starts_with('h'));
+  EXPECT_FALSE(Str.starts_with('i'));
 }
 
 TEST(StringRefTest, StartsWithInsensitive) {
@@ -421,6 +423,8 @@ TEST(StringRefTest, EndsWith) {
   EXPECT_FALSE(Str.ends_with("helloworld"));
   EXPECT_FALSE(Str.ends_with("worldhello"));
   EXPECT_FALSE(Str.ends_with("so"));
+  EXPECT_TRUE(Str.ends_with('o'));
+  EXPECT_FALSE(Str.ends_with('p'));
 }
 
 TEST(StringRefTest, EndsWithInsensitive) {