LSC: Move expensive variables on their last use to avoid copies.

This cl was generated with go/gwp-clang-tidy.
The following lines are helpful in [determining](http://go/gwp-clang-tidy#metrics-summary) the impact of the change:
```
gwp_clang_tidy_metric: 82.28 gcus
gwp_clang_tidy_metric_for_key: 82.28 gcus
gwp_clang_tidy_fn_name: http2::HpackDecoderDynamicTable::Insert(std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>, std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>)
gwp_clang_tidy_num_overlapping_findings: 2
gwp_clang_tidy_call: http2::HpackDecoderDynamicTable::Insert(std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>, std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>) -> http2::HpackStringPair::~HpackStringPair() (5.0135 gcus)
gwp_clang_tidy_call: http2::HpackDecoderDynamicTable::Insert(std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>, std::__u::basic_string<char, std::__u::char_traits<char>, std::__u::allocator<char>>) -> quiche::QuicheCircularDeque<http2::HpackStringPair, 3ul, std::__u::allocator<http2::HpackStringPair>>::push_front(http2::HpackStringPair const&) -> ... -> http2::HpackStringPair::HpackStringPair(http2::HpackStringPair const&) (77.2665 gcus)
gwp_clang_tidy_type_name: http2::HpackStringPair
gwp_clang_tidy_check: google3-runtime-missing-move
```

Tested:
    TAP train for global presubmit queue
    http://test/OCL:639793614:BASE:639779627:1717456875696:32a03877
PiperOrigin-RevId: 640138975