[lldb] fix memory leak in "GetGDBServerRegisterInfoXMLAndProcess"

[lldb] fix memory leak in "GetGDBServerRegisterInfoXMLAndProcess"

While running heap checker on a test that uses LLDB API, the following memory leak is found:

RAW: HeapChecker started...
RAW: Leak check _main_ detected leaks of 34 bytes in 4 objects
RAW: The 2 largest leaks:
RAW: Leak of 17 bytes in 2 objects allocated from:
@ 0x7fb93bd20166 NewHook()
@ 0x7fb929372a73 absl::base_internal::MallocHook::InvokeNewHookSlow()
@ 0x5600d1046093 libc_malloc
@ 0x7fb974529c03 xmlStrdup
@ 0x7fb9744c2a0b xmlGetProp
@ 0x7fb9749d9ed6 lldb_private::XMLNode::GetAttributeValue()
@ 0x7fb979043001 std::u::function::policy_invoker<>::__call_impl<>()
@ 0x7fb9749da06d lldb_private::XMLNode::ForEachChildElement()
@ 0x7fb97903c54d lldb_private::process_gdb_remote::ProcessGDBRemote::GetGDBServerRegisterInfoXMLAndProcess()
@ 0x7fb97902cfe4 lldb_private::process_gdb_remote::ProcessGDBRemote::GetGDBServerRegisterInfo()
@ 0x7fb97902c1d0 lldb_private::process_gdb_remote::ProcessGDBRemote::BuildDynamicRegisterInfo()
@ 0x7fb97902e92a lldb_private::process_gdb_remote::ProcessGDBRemote::SetThreadStopInfo()
@ 0x7fb97902db18 lldb_private::process_gdb_remote::ProcessGDBRemote::DoConnectRemote()
@ 0x7fb97584965e lldb_private::Process::ConnectRemote()
@ 0x7fb975839fa6 lldb_private::Platform::DoConnectProcess()
@ 0x7fb97583a39e lldb_private::Platform::ConnectProcessSynchronous()
@ 0x7fb97545b28b CommandObjectProcessConnect::DoExecute()
@ 0x7fb9755a70c9 lldb_private::CommandObjectParsed::Execute()
@ 0x7fb97559c0e9 lldb_private::CommandInterpreter::HandleCommand()
@ 0x7fb975460145 lldb_private::CommandObjectRegexCommand::DoExecute()
@ 0x7fb9755a72d2 lldb_private::CommandObjectRaw::Execute()
@ 0x7fb97559c0e9 lldb_private::CommandInterpreter::HandleCommand()
@ 0x7fb997a5f22e lldb::SBCommandInterpreter::HandleCommand()
@ 0x7fb997a5ef9b lldb::SBCommandInterpreter::HandleCommand()

This change fixes the memory leaks by freeing memory after it is no
longer in use. Tested with "ninja check-lldb".

Differential revision: https://reviews.llvm.org/D116707