[AST] Fix formatting whitespace-only comments

[refi64]

Summary

If a string consisted only of whitespace, the emptiness check in
getFormattedText would fail, but DropTrailingNewLines would then
call Str.back() on an empty string. In practice, this can easily
crash clangd if an editor asks for a declaration comment that is
whitespace-only.

[refi64]

(I manually ran clang-format-diff afterwards, there were no suggested changes.)

clang/lib/AST/RawCommentList.cpp

 //===--- RawCommentList.cpp - Processing raw comments -----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
for (L.lex(Tok); Tok.isNot(comments::tok::eof); L.lex(Tok)) {
       }
       Result += L.getSpelling(Tok, SourceMgr);
     }
     // We've reached the end of file token.
     return false;
   };
 
   auto DropTrailingNewLines = [](std::string &Str) {
-    while (Str.back() == '\n')
+    while (!Str.empty() && Str.back() == '\n')
       Str.pop_back();
   };
 
   // Process first line separately to remember indent for the following lines.
   if (!LexLine(/*IsFirstLine=*/true)) {
     DropTrailingNewLines(Result);
     return Result;
   }
   // Process the rest of the lines.
   while (LexLine(/*IsFirstLine=*/false))
     ;
   DropTrailingNewLines(Result);
   return Result;
 }

clang/unittests/AST/CommentTextTest.cpp

 //===- unittest/AST/CommentTextTest.cpp - Comment text extraction test ----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 R"cpp(
 /// \brief This is the brief part of the comment.
 /// \param a something about a.
 /// @param b something about b.)cpp");
   EXPECT_EQ(ExpectedOutput, Formatted);
   // clang-format on
 }
 
+TEST_F(CommentTextTest, WorksWithEmptyBlockComments) {
+  auto ExpectedOutput = "";
+  auto Formatted = formatComment(R"(/* */)");
+  EXPECT_EQ(ExpectedOutput, Formatted);
+}
+
 } // namespace clang