[ADT] [NFC] Introduce isLower and isUpper helpers.

[jacek]

Summary

For D149095.

[MaskRay]

We may add a unittest to unittests/ADT/StringExtrasTest.cpp, but this may not be worth the trouble as isLower/etc are used by others we already provide (e.g. toUpper).

[jhenderson]

LGTM. I'd add the unit test, but I'm not going to push strongly for it.

llvm/include/llvm/ADT/StringExtras.h

104

[jacek]

I pushed with tests, thanks.

llvm/include/llvm/ADT/StringExtras.h

 }
 
 /// Checks if character \p C is one of the 10 decimal digits.
 inline bool isDigit(char C) { return C >= '0' && C <= '9'; }
 
 /// Checks if character \p C is a hexadecimal numeric character.
 inline bool isHexDigit(char C) { return hexDigitValue(C) != ~0U; }
 
+/// Checks if character \p C is a lowercase letter as classified by "C" locale.
+inline bool isLower(char C) { return 'a' <= C && C <= 'z'; }
+
+/// Checks if character \p C is a uppercase letter as classified by "C" locale.
+inline bool isUpper(char C) { return 'A' <= C && C <= 'Z'; }
+
 /// Checks if character \p C is a valid letter as classified by "C" locale.
-inline bool isAlpha(char C) {
-  return ('a' <= C && C <= 'z') || ('A' <= C && C <= 'Z');
-}
+inline bool isAlpha(char C) { return isLower(C) || isUpper(C); }
 
 /// Checks whether character \p C is either a decimal digit or an uppercase or
 /// lowercase letter as classified by "C" locale.
 inline bool isAlnum(char C) { return isAlpha(C) || isDigit(C); }
 
 /// Checks whether character \p C is valid ASCII (high bit is zero).
 inline bool isASCII(char C) { return static_cast<unsigned char>(C) <= 127; }
 

 /// Locale-independent version of the C standard library isspace.
 inline bool isSpace(char C) {
   return C == ' ' || C == '\f' || C == '\n' || C == '\r' || C == '\t' ||
          C == '\v';
 }
 
 /// Returns the corresponding lowercase character if \p x is uppercase.
 inline char toLower(char x) {
-  if (x >= 'A' && x <= 'Z')
+  if (isUpper(x))
     return x - 'A' + 'a';
   return x;
 }
 
 /// Returns the corresponding uppercase character if \p x is lowercase.
 inline char toUpper(char x) {
-  if (x >= 'a' && x <= 'z')
+  if (isLower(x))
     return x - 'a' + 'A';
   return x;
 }
 
 inline std::string utohexstr(uint64_t X, bool LowerCase = false,
                              unsigned Width = 0) {
   char Buffer[17];
   char *BufPtr = std::end(Buffer);