`SharedCache::ParseExportTrie` is getting called a lot during DSC library loading and analysis. In large part due to the hot path `SharedCache::FindSymbolAtAddrAndApplyToAddr`. Its unnecessary for it to be being called more than once per DSC header as the export list symbol information is stored in `SharedCache::m_exportInfos`.

This commit adds the function `SharedCache::GetExportListForHeader`, which will either return the header's list of symbol information cached in `SharedCache::m_exportInfos` or call `SharedCache::ParseExportTrie` and cache the results in `SharedCache::m_exportInfos`.

This should also improve the execution time of `SharedCache::LoadAllSymbolsAndWait`.

Further improvement here would be to add locking to `SharedCache::GetExportListForHeader` so that races don't result in redundant parsing of the export trie for the same header if multiple threads call `SharedCache::GetExportListForHeader` at the same time for the same header. This only really matters during initial loading because from what I can tell that parses all the export trie's anyway.

…ck if `m_exportInfos` was modified

This probably makes more sense than the current solution of using execution of the callback parameter to determine if `m_exportInfos` was modified.

This commit changes 2 things;
1. `m_exportInfos` is now a map where its values are also a map rather than a vector of pairs. The reason for this is that `SharedCache::FindSymbolAtAddrAndApplyToAddr` is a hot path which does by far the most accesses to `m_exportInfos`. In that function it must find the correct symbol for a given address so a map lookup will be much quicker than iterating a vector. The other use cases of `m_exportInfos` would prefer a vector but they are executed very infrequently.
2. The symbols are stored in `m_exportInfos` as references to the `Symbol` type. This makes more sense because otherwise there is a lot of time spent converting to and from a `Symbol` type and a pair of `BNSymbolType` + a `std::string`.

…::FindSymbolAtAddrAndApplyToAddr` if a symbol is found

This avoids expensive copying when returning a value from the map in `SharedCache::GetExportListForHeader`. Additionally it ensures that the value stays alive and at the same location in memory if `m_exportInfos` is modified and requires its storage to be re-allocated.

I was unable to use a `unique_ptr` instead of a `shared_ptr` because of copy semantics with `m_exportInfos` in `ViewStateCacheStore`. I don't see things being any worse using `shared_ptr` instead of `unique_ptr` anyway and it means less code changes.