[clang-format] Fix an assertion failure on comment-only config files (llvm#163111)

(cherry picked from commit 059f2df)

Author: owenca

clang/lib/Format/Format.cpp

@@ -2132,8 +2132,9 @@ std::error_code parseConfiguration(llvm::MemoryBufferRef Config,
   Input >> Styles;
   if (Input.error())
     return Input.error();
+  if (Styles.empty())
+    return make_error_code(ParseError::Success);
 
-  assert(!Styles.empty());
   const auto StyleCount = Styles.size();
 
   // Start from the second style as (only) the first one may be the default.

clang/unittests/Format/ConfigParseTest.cpp

@@ -1249,6 +1249,13 @@ TEST(ConfigParseTest, ParsesConfigurationWithLanguages) {
               IndentWidth, 56u);
 }
 
+TEST(ConfigParseTest, AllowCommentOnlyConfigFile) {
+  FormatStyle Style = {};
+  Style.Language = FormatStyle::LK_Cpp;
+  EXPECT_EQ(parseConfiguration("#Language: C", &Style), ParseError::Success);
+  EXPECT_EQ(Style.Language, FormatStyle::LK_Cpp);
+}
+
 TEST(ConfigParseTest, AllowCppForC) {
   FormatStyle Style = {};
   Style.Language = FormatStyle::LK_C;