[clang-tidy] Don't split \r\n in modernize-use-std-print check

[mikecrowe]

Summary

When given:
printf("Hello\r\n");

it's clearer to leave the CRLF intact and convert this to:
std::print("Hello\r\n");

than to remove the trailing newline and convert it to:
std::println("Hello\r");

Update the documentation to match, and clarify the situations for using
println vs print which weren't previously explained.

[PiotrZSL]

LGTM

clang-tools-extra/clang-tidy/utils/FormatStringConverter.cpp

 /// Called at the very end just before applying fixes to capture the last part
 /// of the format string.
 void FormatStringConverter::finalizeFormatText() {
   appendFormatText(
       StringRef(PrintfFormatString.begin() + PrintfFormatStringPos,
                 PrintfFormatString.size() - PrintfFormatStringPos));
   PrintfFormatStringPos = PrintfFormatString.size();
 
+  // It's clearer to convert printf("Hello\r\n"); to std::print("Hello\r\n")
+  // than to std::println("Hello\r");
   if (StringRef(StandardFormatString).ends_with("\\n") &&
-      !StringRef(StandardFormatString).ends_with("\\\\n")) {
+      !StringRef(StandardFormatString).ends_with("\\\\n") &&
+      !StringRef(StandardFormatString).ends_with("\\r\\n")) {
     UsePrintNewlineFunction = true;
     FormatStringNeededRewriting = true;
     StandardFormatString.erase(StandardFormatString.end() - 2,
                                StandardFormatString.end());
   }
 
   StandardFormatString.push_back('\"');
 }

clang-tools-extra/docs/clang-tidy/checks/modernize/use-std-print.rst

 
 If the call is deemed suitable for conversion then:
 
 - ``printf``, ``fprintf``, ``absl::PrintF``, ``absl::FPrintF`` and any
   functions specified by the `PrintfLikeFunctions` option or
   `FprintfLikeFunctions` are replaced with the function specified by the
   `ReplacementPrintlnFunction` option if the format string ends with ``\n``
   or `ReplacementPrintFunction` otherwise.
-- the format string is rewritten to use the ``std::formatter`` language and
-  a ``\n`` is removed from the end.
+- the format string is rewritten to use the ``std::formatter`` language. If
+  a ``\n`` is found at the end of the format string not preceded by ``r``
+  then it is removed and `ReplacementPrintlnFunction` is used rather than
+  `ReplacementPrintFunction`.
 - any arguments that corresponded to ``%p`` specifiers that
   ``std::formatter`` wouldn't accept are wrapped in a ``static_cast``
   to ``const void *``.
 - any arguments that corresponded to ``%s`` specifiers where the argument
   is of ``signed char`` or ``unsigned char`` type are wrapped in a
   ``reinterpret_cast<const char *>``.
 - any arguments where the format string and the parameter differ in
   signedness will be wrapped in an approprate ``static_cast`` if `StrictMode`

clang-tools-extra/test/clang-tidy/checkers/modernize/use-std-print.cpp

void printf_deceptive_newline() {
   // CHECK-MESSAGES: [[@LINE-1]]:3: warning: use 'std::print' instead of 'printf' [modernize-use-std-print]
   // CHECK-FIXES: std::print("Hello\\n");
 
   printf("Hello\x0a");
   // CHECK-MESSAGES: [[@LINE-1]]:3: warning: use 'std::println' instead of 'printf' [modernize-use-std-print]
   // CHECK-FIXES: std::println("Hello");
 }
 
+void printf_crlf_newline() {
+  printf("Hello\r\n");
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: use 'std::print' instead of 'printf' [modernize-use-std-print]
+  // CHECK-FIXES: std::print("Hello\r\n");
+
+  printf("Hello\r\\n");
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: use 'std::print' instead of 'printf' [modernize-use-std-print]
+  // CHECK-FIXES: std::print("Hello\r\\n");
+}
+
 // std::print returns nothing, so any callers that use the return
 // value cannot be automatically translated.
 int printf_uses_return_value(int choice) {
   const int i = printf("Return value assigned to variable %d\n", 42);
 
   extern void accepts_int(int);
   accepts_int(printf("Return value passed to function %d\n", 42));