[lldb] Make sure completions don't end with newlines (llvm#117054)

The logic that prints completions and their descriptions assumes neither
the completion itself nor the description ends with a newline. I
considered making this an assert, but decided against it as completions
can indirectly come from user input (e.g. oddly crafted names). Instead,
avoid the potential for mistakes by defensively stripping them.

(cherry picked from commit 4b5a8d6)

Author: JDevlieghere

lldb/include/lldb/Utility/CompletionRequest.h

@@ -52,8 +52,8 @@ class CompletionResult {
   public:
     Completion(llvm::StringRef completion, llvm::StringRef description,
                CompletionMode mode)
-        : m_completion(completion.str()), m_descripton(description.str()),
-          m_mode(mode) {}
+        : m_completion(completion.rtrim().str()),
+          m_descripton(description.rtrim().str()), m_mode(mode) {}
     const std::string &GetCompletion() const { return m_completion; }
     const std::string &GetDescription() const { return m_descripton; }
     CompletionMode GetMode() const { return m_mode; }