[lldb-vscode] Fix dangling pointer in request_evaluate.

[jgorbe]

Summary

SBError::GetCString() returns a pointer to a string owned by the SBError
object. The code here was calling GetCString on a temporary and using
the returned pointer after the temporary was destroyed.

[zturner]

Another option would be to just assign it to a std::string, but this is fine too.

[jgorbe]

I considered assigning it to a std::string, but GetCString() can return a null pointer and I'm not sure you can construct a std::string directly from that.

lldb/tools/lldb-vscode/lldb-vscode.cpp

if (!expression.empty() && expression[0] == '`') {
     // parser. "frame variable" is more reliable than the expression parser in
     // many cases and it is faster.
     lldb::SBValue value = frame.GetValueForVariablePath(
         expression.data(), lldb::eDynamicDontRunTarget);
     if (value.GetError().Fail())
       value = frame.EvaluateExpression(expression.data());
     if (value.GetError().Fail()) {
       response["success"] = llvm::json::Value(false);
-      const char *error_cstr = value.GetError().GetCString();
+      // This error object must live until we're done with the pointer returned
+      // by GetCString().
+      lldb::SBError error = value.GetError();
+      const char *error_cstr = error.GetCString();
       if (error_cstr && error_cstr[0])
         EmplaceSafeString(response, "message", std::string(error_cstr));
       else
         EmplaceSafeString(response, "message", "evaluate failed");
     } else {
       SetValueForKey(value, body, "result");
       auto value_typename = value.GetType().GetDisplayTypeName();
       EmplaceSafeString(body, "type", value_typename ? value_typename : NO_TYPENAME);