[ELF] Simplify isValidCIdentifier. NFC

lld/Common/Strings.cpp

@@ -76,9 +76,8 @@ std::vector<uint8_t> lld::parseHex(StringRef s) {
 
 // Returns true if S is valid as a C language identifier.
 bool lld::isValidCIdentifier(StringRef s) {
-  return !s.empty() && (isAlpha(s[0]) || s[0] == '_') &&
-         std::all_of(s.begin() + 1, s.end(),
-                     [](char c) { return c == '_' || isAlnum(c); });
+  return !s.empty() && !isDigit(s[0]) &&
+         llvm::all_of(s, [](char c) { return isAlnum(c) || c == '_'; });
 }
 
 // Write the contents of the a buffer to a file