Convert diagnostics about multi-character literals from extension to warning

This addresses PR46797.

Author: Chuyang Chen

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -104,10 +104,10 @@ def warn_cxx98_compat_raw_string_literal : Warning<
   "raw string literals are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 
-def ext_multichar_character_literal : ExtWarn<
+def warn_multichar_character_literal : Warning<
   "multi-character character constant">, InGroup<MultiChar>;
-def ext_four_char_character_literal : Extension<
-  "multi-character character constant">, InGroup<FourByteMultiChar>;
+def warn_four_char_character_literal : Warning<
+  "multi-character character constant">, InGroup<FourByteMultiChar>, DefaultIgnore;
 
 
 // Unicode and UCNs

clang/lib/Lex/LiteralSupport.cpp

@@ -1373,9 +1373,9 @@ CharLiteralParser::CharLiteralParser(const char *begin, const char *end,
     if (isWide())
       PP.Diag(Loc, diag::warn_extraneous_char_constant);
     else if (isAscii() && NumCharsSoFar == 4)
-      PP.Diag(Loc, diag::ext_four_char_character_literal);
+      PP.Diag(Loc, diag::warn_four_char_character_literal);
     else if (isAscii())
-      PP.Diag(Loc, diag::ext_multichar_character_literal);
+      PP.Diag(Loc, diag::warn_multichar_character_literal);
     else
       PP.Diag(Loc, diag::err_multichar_utf_character_literal);
     IsMultiChar = true;

clang/test/Lexer/constants.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -pedantic -ftrigraphs %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wfour-char-constants -pedantic -ftrigraphs %s
 
 int x = 000000080;  // expected-error {{invalid digit}}
 

clang/test/Lexer/multi-char-constants.c

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wfour-char-constants -pedantic-errors %s
+
+int x = 'ab'; // expected-warning {{multi-character character constant}}
+int y = 'abcd'; // expected-warning {{multi-character character constant}}