Bug 314169 – [Build Speed] Split GeneratedSerializers.mm by source domain for parallel compile

• Component: New Bugs
• Status: RESOLVED FIXED

[zak ridouh]

generate-serializers.py emitted a single GeneratedSerializers.mm
(~3.9 MB, ~61s compile) that gated the WebKit build's critical path.
Partition it by the top-level Source/WebKit/<DIR>/ directory of each
input .serialization.in, producing eight TUs that compile in parallel:

  Shared                 35.2s   (2583 specializations)
  GPUProcess        14.9s   (81)
  NetworkProcess 14.7s   (24)
  WebProcess        14.5s   (55)
  UIProcess            14.3s   (0)
  Common              14.2s   (18)  residual bucket
  Platform               14.0s   (30)
  ModelProcess     13.0s    (2)

Critical-path edge: ~61s -> 35.2s. Total specialization count is
preserved exactly (2793 ArgumentCoder<T>, matching the prior file).
Cross-TU references resolve at link time. 

The Common bucket catches types whose .serialization.in lives outside
Source/WebKit/<DIR>/ (WebCore cross-framework types and the generated
SharedPreferencesForWebProcess); the generator lists them on stderr
so future additions stay visible in review.

[Radar WebKit Bug Importer]

<rdar://problem/176329962>

[zak ridouh]

Pull request: https://github.com/WebKit/WebKit/pull/64344

[EWS]

Committed 312747@main (9823057a1830): <https://commits.webkit.org/312747@main>

Reviewed commits have been landed. Closing PR #64344 and removing active labels.