[clang-format] Handle C digit separators (llvm#158418)

Fixes llvm#158413

(cherry picked from commit 30f4781)

Author: owenca

clang/lib/Format/Format.cpp

@@ -4043,6 +4043,7 @@ LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   switch (Style.Language) {
   case FormatStyle::LK_C:
     LangOpts.C11 = 1;
+    LangOpts.C23 = 1;
     break;
   case FormatStyle::LK_Cpp:
   case FormatStyle::LK_ObjC:

clang/unittests/Format/TokenAnnotatorTest.cpp

@@ -4097,6 +4097,13 @@ TEST_F(TokenAnnotatorTest, UTF8StringLiteral) {
   EXPECT_TOKEN(Tokens[1], tok::utf8_string_literal, TT_Unknown);
 }
 
+TEST_F(TokenAnnotatorTest, C23DigitSeparator) {
+  auto Tokens = annotate("return 1'000;", getLLVMStyle(FormatStyle::LK_C));
+  ASSERT_EQ(Tokens.size(), 4u) << Tokens;
+  EXPECT_EQ(Tokens[1]->TokenText, "1'000");
+  EXPECT_TOKEN(Tokens[2], tok::semi, TT_Unknown);
+}
+
 TEST_F(TokenAnnotatorTest, IdentifierPackage) {
   auto Tokens = annotate("auto package;");
   ASSERT_EQ(Tokens.size(), 4u) << Tokens;