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

[obruns]

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

[teemperor]

We don't usually use alternative operator spelling in Clang, but otherwise this LGTM. I'll land this for you.

[teemperor]

Landed as ad7211df6f257e39da2e5a11b2456b4488f32a1e

Btw, usually patches also need a test (especially if they are less trivial than this one).

clang/lib/AST/RawCommentList.cpp

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;
 }