Adjust batch partition count calculation

[kastiglione]

This mirrors the batch partition calculation change in swiftlang/swift#28972. See below for an explanation.

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While exploring how to optimize global build times, I noticed that the batch sizing logic wasn't producing multiple batches until a module had 50 files. Based on @graydon's detailed batch calculation comment (which says "cap the batch size ... at ... 25"), and based on the variable names ("SizeLimit"), I believe the batch calculation implementation is off.

The simple example is a module of 26 files. With the existing batch calculation, the result is a 26/25=1 batch. However this is larger than the DefaultSizeLimit. This calculation continues to yield 1 for modules up to 49 files. Only at modules of 50 files does the batch calculation produce two batches.

Here's a table showing the effects of the current partition count calculation:

Num Files	Num Batches	Min Batch Size	Max Batch Size
1-49	1	1	49
50-74	2	25	37
75-99	3	25	33
100-124	4	25	31

This table shows that the max batch size is inversely proportional with the number of files. Smaller modules have a higher max batch size, and as a module gets larger, its max batch size trends toward 25. It also shows that after 50, the minimum batch size is consistently 25.

With this proposed change to the batch count calculation, the math is now:

Num Files	Num Batches	Min Batch Size	Max Batch Size
1-25	1	1	25
26-50	2	13	25
51-75	3	17	25
76-100	4	19	25
101-125	5	21	25

This table shows that the batch size is in fact capped to 25, independent of how many files are in the module. It also show the minimum batch size increases with module size.

[DougGregor]

@swift-ci please test