Set a higher LRU cache size for makeCheapPathFunction entries.

In PR #10720, we introduced the makeCheapPathFunction in an effort to
reduce the caching overhead for very frequently called (and already pretty
quick) operations like files.stat.

However, the default maximum LRU cache size of Math.pow(2, 16) can cause
quite a bit of cache eviction churn for large applications, which @veered
has identified as a potential source of build performance problems.

By setting the maximum cache size to Math.pow(2, 20) instead, I am no
longer seeing any files.stat calls in the profiling output for rebuilding
a large internal app, saving several seconds of rebuild time. The obvious
downside is that this cache might accumulate more memory over time, which
is why I didn't just set the max to Infinity, though that might be a
viable option if the total set of paths ever stat'd is small enough to fit
into the available memory.

In the future, I hope to find ways of managing LRU cache size that respond
to actual memory pressure (relative to available memory), rather than
pruning the cache after an arbitrary numeric threshold is reached.

tools/fs/optimistic.ts

@@ -229,6 +229,11 @@ function makeCheapPathFunction<TResult>(
     return pathFunction;
   }
   const wrapper = wrap(pathFunction, {
+    // The maximum LRU cache size is Math.pow(2, 16) by default, but it's
+    // important to prevent eviction churn for very-frequently-called
+    // functions like optimisticStatOrNull. While it's tempting to set
+    // this limit to Infinity, increasing it by 16x comes close enough.
+    max: Math.pow(2, 20),
     subscribe(path) {
       let watcher: SafeWatcher | null = watch(
         path,