[clang] fix undefined behaviour in RawComment::getFormattedText()

Summary:
Calling `back()` and `pop_back()` on the empty string is undefined
behavior [1,2].

The issue manifested itself as an uncaught `std::out_of_range` exception
when running `clangd` compiled on RHEL7 using devtoolset-9.

[1] https://en.cppreference.com/w/cpp/string/basic_string/back
[2] https://en.cppreference.com/w/cpp/string/basic_string/pop_back

Fixes: 1ff7c32fc91c607b690d4bb9cf42f406be8dde68

Reviewers: teemperor, ioeric, cfe-commits

Reviewed By: teemperor

Subscribers: ilya-biryukov, kadircet, usaxena95

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77468

clang/lib/AST/RawCommentList.cpp

@@ -431,7 +431,7 @@ std::string RawComment::getFormattedText(const SourceManager &SourceMgr,
   };
 
   auto DropTrailingNewLines = [](std::string &Str) {
-    while (Str.back() == '\n')
+    while (!Str.empty() && Str.back() == '\n')
       Str.pop_back();
   };