[ADT] Add more ArrayRef <-> StringRef conversion functions

[steven_wu]

Summary

Add new functions in StringExtras to convert byte size type array to
StringRef and vice versa.

[dexonsmith]

LGTM.

llvm/include/llvm/ADT/StringExtras.h

 
 /// Construct a string ref from a boolean.
 inline StringRef toStringRef(bool B) { return StringRef(B ? "true" : "false"); }
 
 /// Construct a string ref from an array ref of unsigned chars.
 inline StringRef toStringRef(ArrayRef<uint8_t> Input) {
   return StringRef(reinterpret_cast<const char *>(Input.begin()), Input.size());
 }
+inline StringRef toStringRef(ArrayRef<char> Input) {
+  return StringRef(Input.begin(), Input.size());
+}
 
 /// Construct a string ref from an array ref of unsigned chars.
-inline ArrayRef<uint8_t> arrayRefFromStringRef(StringRef Input) {
-  return {Input.bytes_begin(), Input.bytes_end()};
+template <class CharT = uint8_t>
+inline ArrayRef<CharT> arrayRefFromStringRef(StringRef Input) {
+  static_assert(std::is_same<CharT, char>::value ||
+                    std::is_same<CharT, unsigned char>::value ||
+                    std::is_same<CharT, signed char>::value,
+                "Expected byte type");
+  return ArrayRef<CharT>(reinterpret_cast<const CharT *>(Input.data()),
+                         Input.size());
 }
 
 /// Interpret the given character \p C as a hexadecimal digit and return its
 /// value.
 ///
 /// If \p C is not a valid hex digit, -1U is returned.
 inline unsigned hexDigitValue(char C) {
   /* clang-format off */

llvm/unittests/ADT/StringExtrasTest.cpp

 }
 
 TEST(StringExtrasTest, splitCharForLoop) {
   llvm::SmallVector<StringRef, 4> Result;
   for (StringRef x : split("foo,bar,,baz", ','))
     Result.push_back(x);
   EXPECT_THAT(Result, testing::ElementsAre("foo", "bar", "", "baz"));
 }
+
+TEST(StringExtrasTest, arrayToStringRef) {
+  auto roundTripTestString = [](llvm::StringRef Str) {
+    EXPECT_EQ(Str, toStringRef(arrayRefFromStringRef<uint8_t>(Str)));
+    EXPECT_EQ(Str, toStringRef(arrayRefFromStringRef<char>(Str)));
+  };
+  roundTripTestString("");
+  roundTripTestString("foo");
+  roundTripTestString("\0\n");
+  roundTripTestString("\xFF\xFE");
+}