[lldb] Constify methods in CommandReturnObject (NFC)

There's no reason these methods cannot be `const`. Currently this
prevents us from passing around a const ref. This patch is in
preparation for llvm#125006.

(cherry picked from commit 7469032)

Author: JDevlieghere

lldb/include/lldb/Interpreter/CommandReturnObject.h

@@ -32,9 +32,9 @@ class CommandReturnObject {
   ~CommandReturnObject() = default;
 
   /// Format any inline diagnostics with an indentation of \c indent.
-  std::string GetInlineDiagnosticString(unsigned indent);
+  std::string GetInlineDiagnosticString(unsigned indent) const;
 
-  llvm::StringRef GetOutputString() {
+  llvm::StringRef GetOutputString() const {
     lldb::StreamSP stream_sp(m_out_stream.GetStreamAtIndex(eStreamStringIndex));
     if (stream_sp)
       return std::static_pointer_cast<StreamString>(stream_sp)->GetString();
@@ -46,7 +46,7 @@ class CommandReturnObject {
   /// If \c with_diagnostics is true, all diagnostics are also
   /// rendered into the string. Otherwise the expectation is that they
   /// are fetched with \ref GetInlineDiagnosticString().
-  std::string GetErrorString(bool with_diagnostics = true);
+  std::string GetErrorString(bool with_diagnostics = true) const;
   StructuredData::ObjectSP GetErrorData();
 
   Stream &GetOutputStream() {
@@ -95,11 +95,11 @@ class CommandReturnObject {
     m_err_stream.SetStreamAtIndex(eImmediateStreamIndex, stream_sp);
   }
 
-  lldb::StreamSP GetImmediateOutputStream() {
+  lldb::StreamSP GetImmediateOutputStream() const {
     return m_out_stream.GetStreamAtIndex(eImmediateStreamIndex);
   }
 
-  lldb::StreamSP GetImmediateErrorStream() {
+  lldb::StreamSP GetImmediateErrorStream() const {
     return m_err_stream.GetStreamAtIndex(eImmediateStreamIndex);
   }
 

lldb/include/lldb/Utility/StreamTee.h

@@ -85,7 +85,7 @@ class StreamTee : public Stream {
     return result;
   }
 
-  lldb::StreamSP GetStreamAtIndex(uint32_t idx) {
+  lldb::StreamSP GetStreamAtIndex(uint32_t idx) const {
     lldb::StreamSP stream_sp;
     std::lock_guard<std::recursive_mutex> guard(m_streams_mutex);
     if (idx < m_streams.size())

lldb/source/Interpreter/CommandReturnObject.cpp

@@ -123,7 +123,8 @@ void CommandReturnObject::SetError(llvm::Error error) {
   }
 }
 
-std::string CommandReturnObject::GetInlineDiagnosticString(unsigned indent) {
+std::string
+CommandReturnObject::GetInlineDiagnosticString(unsigned indent) const {
   StreamString diag_stream(m_colors);
   RenderDiagnosticDetails(diag_stream, indent, true, m_diagnostics);
   // Duplex the diagnostics to the secondary stream (but not inlined).
@@ -133,7 +134,7 @@ std::string CommandReturnObject::GetInlineDiagnosticString(unsigned indent) {
   return diag_stream.GetString().str();
 }
 
-std::string CommandReturnObject::GetErrorString(bool with_diagnostics) {
+std::string CommandReturnObject::GetErrorString(bool with_diagnostics) const {
   StreamString stream(m_colors);
   if (with_diagnostics)
     RenderDiagnosticDetails(stream, std::nullopt, false, m_diagnostics);