[Support] Fix -Wpessimizing-move in Mustache.cpp (NFC)

/llvm-project/llvm/lib/Support/Mustache.cpp:299:27:
error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]
    PrevToken.TokenBody = std::move(Unindented.str());
                          ^
/llvm-project/llvm/lib/Support/Mustache.cpp:299:27: note: remove std::move call here
    PrevToken.TokenBody = std::move(Unindented.str());
                          ^~~~~~~~~~                ~
1 error generated.

Author: DamonFool

llvm/lib/Support/Mustache.cpp

@@ -296,7 +296,7 @@ void stripTokenBefore(SmallVectorImpl<Token> &Tokens, size_t Idx,
   StringRef Unindented = PrevTokenBody.rtrim(" \r\t\v");
   size_t Indentation = PrevTokenBody.size() - Unindented.size();
   if (CurrentType != Token::Type::Partial)
-    PrevToken.TokenBody = std::move(Unindented.str());
+    PrevToken.TokenBody = Unindented.str();
   CurrentToken.setIndentation(Indentation);
 }