Fix a use-after-free in GDBRemoteCommunicationServerLLGS

Although it's called "GetString", StreamString::GetString actually
returns a StringRef. Creating a json object with a StringRef does not
make a copy, which means the StringRef will be dangling as soon as the
underlying stream is destroyed. Add a .str() to force the json object to
hold a copy of the string.

This fixes nearly every test on linux.

llvm-svn: 373572

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp

@@ -462,7 +462,8 @@ GetRegistersAsJSON(NativeThreadProtocol &thread) {
     WriteRegisterValueInHexFixedWidth(stream, reg_ctx, *reg_info_p,
                                       &reg_value, lldb::eByteOrderBig);
 
-    register_object.try_emplace(llvm::to_string(reg_num), stream.GetString());
+    register_object.try_emplace(llvm::to_string(reg_num),
+                                stream.GetString().str());
   }
 
   return register_object;