Mark inline functions with only a simple comparison in strings/ascii.h as constexpr.

PiperOrigin-RevId: 686814718
Change-Id: Ia712f4cd24ebf3d02fd0b03cd6973bb53e128682

absl/strings/ascii.h

@@ -139,32 +139,42 @@ inline bool ascii_isxdigit(unsigned char c) {
 //
 // Determines whether the given character can be represented as a decimal
 // digit character (i.e. {0-9}).
-inline bool ascii_isdigit(unsigned char c) { return c >= '0' && c <= '9'; }
+inline constexpr bool ascii_isdigit(unsigned char c) {
+  return c >= '0' && c <= '9';
+}
 
 // ascii_isprint()
 //
 // Determines whether the given character is printable, including spaces.
-inline bool ascii_isprint(unsigned char c) { return c >= 32 && c < 127; }
+inline constexpr bool ascii_isprint(unsigned char c) {
+  return c >= 32 && c < 127;
+}
 
 // ascii_isgraph()
 //
 // Determines whether the given character has a graphical representation.
-inline bool ascii_isgraph(unsigned char c) { return c > 32 && c < 127; }
+inline constexpr bool ascii_isgraph(unsigned char c) {
+  return c > 32 && c < 127;
+}
 
 // ascii_isupper()
 //
 // Determines whether the given character is uppercase.
-inline bool ascii_isupper(unsigned char c) { return c >= 'A' && c <= 'Z'; }
+inline constexpr bool ascii_isupper(unsigned char c) {
+  return c >= 'A' && c <= 'Z';
+}
 
 // ascii_islower()
 //
 // Determines whether the given character is lowercase.
-inline bool ascii_islower(unsigned char c) { return c >= 'a' && c <= 'z'; }
+inline constexpr bool ascii_islower(unsigned char c) {
+  return c >= 'a' && c <= 'z';
+}
 
 // ascii_isascii()
 //
 // Determines whether the given character is ASCII.
-inline bool ascii_isascii(unsigned char c) { return c < 128; }
+inline constexpr bool ascii_isascii(unsigned char c) { return c < 128; }
 
 // ascii_tolower()
 //