Jan Svoboda 6c6351ee35 [clang][modules] Optimize construction and usage of the submodule index (#113391) ...

This patch avoids eagerly populating the submodule index on `Module`
construction. The `StringMap` allocation shows up in my profiles of
`clang-scan-deps`, while the index is not necessary most of the time. We
still construct it on-demand.

Moreover, this patch avoids performing qualified submodule lookup in
`ASTReader` whenever we're serializing a module graph whose top-level
module is unknown. This is pointless, since that's guaranteed to never
find any existing submodules anyway.

This speeds up `clang-scan-deps` by ~0.5% on my workload.

2024-10-28 11:47:59 -07:00

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