[clang] NFC: test for undefined behaviour in RawComment::getFormattedText()

This diff adds testcase for the issue fixed in https://reviews.llvm.org/D77468
but regression test was not added in the diff. On Clang 9 it caused
crash in cland during code completion.

Test Plan: check-clang-unit

Differential Revision: https://reviews.llvm.org/D103722

clang/unittests/AST/CommentTextTest.cpp

@@ -124,4 +124,11 @@ R"cpp(
   // clang-format on
 }
 
+TEST_F(CommentTextTest, EmptyFormattedText) {
+  // Test that empty formatted text doesn't cause crash.
+  const char *ExpectedOutput = "";
+  auto Formatted = formatComment("//!<");
+  EXPECT_EQ(ExpectedOutput, Formatted);
+}
+
 } // namespace clang