[SwiftLexicalLookup] Unqualified lookup caching

[MAJKFL]

This PR introduces optional caching support to SwiftLexicalLookup. In order to use it, clients can pass an instance of LookupCache as a parameter to the lookup function.

LookupCache keeps track of cache member hits. In order to prevent the cache from taking too much memory, clients can call the LookupCache.evictEntriesWithoutHit function to remove members without a hit and reset the hit property for the remaining members. Calling this function every time after lookup effectively maintains one path from a leaf to the root of the scope tree in cache.

Clients can also optionally set the drop value:

/// Creates a new unqualified lookup cache.
/// `drop` parameter specifies how many eviction calls will be
/// ignored before evicting not-hit members of the cache.
///
/// Example cache eviction sequences (s - skip, e - evict):
/// - `drop = 0` - `e -> e -> e -> e -> e -> ...`
/// - `drop = 1` - `s -> e -> s -> s -> e -> ...`
/// - `drop = 3` - `s -> s -> s -> e -> s -> ...`
///
/// - Note: `drop = 0` effectively maintains exactly one path of cached results to
/// the root in the cache (assuming we evict cache members after each lookup in a sequence of lookups).
/// Higher the `drop` value, more such paths can potentially be stored in the cache at any given moment.
/// Because of that, a higher `drop` value also translates to a higher number of cache-hits,
/// but it might not directly translate to better performance. Because of a larger memory footprint,
/// memory accesses could take longer, slowing down the eviction process. That's why the `drop` value
/// could be fine-tuned to maximize the performance given file size,
/// number of lookups, and amount of available memory.
public init(drop: Int = 0) {
  self.dropMod = drop + 1
}

[MAJKFL]

swiftlang/swift#81209

@swift-ci Please test