Add support for explicit/non-auto pch files

[sean-mcmanus]

If we had a setting to an explicit (build system) pch file then IntelliSense can be configured to re-use the pch file, saving disk and memory usage across all the translation units that use the pch file. VS implements this, but we only support "auto pch" which doesn't share data across TUs.

We could add a setting for this and/or pick it up from compile_commands.json or a custom configuration provider (external build system).

[github-actions]

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[github-actions]

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