Add usage count to find-all-symbols.
FindAllSymbols now finds (most!) main-file usages of the discovered symbols.
The per-TU map output provides Signals: Seen (was NumOccurrences) and Used.
These are no longer part of the SymbolInfo. They are provided in batches, deduplicated.
The reducer aggregates these to find the number of files that use a symbol.
The idea here is to use Signals.Used for ranking: intuitively number of files that
use a symbol is more meaningful than number of files that include the header.