[NFCI][WPD]Use unique string saver to store type id (#106932)

Currently, both
[TypeIdMap](https://github.com/llvm/llvm-project/blob/67a1fdb014790a38a205d28e1748634de34471dd/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1356)
and
[TypeIdCompatibleVtableMap](https://github.com/llvm/llvm-project/blob/67a1fdb014790a38a205d28e1748634de34471dd/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1363)
keep type-id as `std::string` in the combined index for LTO indexing
analysis.

With this change, index uses a unique-string-saver to own the string
copies and two maps above can use string references to save some memory.

This shows a 3% memory reduction (from 8.2GiB to 7.9GiB) in an internal
binary with high indexing memory usage.