Use pre-compiled headers when building Qt with cmake

Some modules define their own manually-maintained lists, and we can rely
on the headers generated by each module to include in the pch as well
e.g. QtCore/QtCore.

There's also e.g. QtWidgetDepends for QtWidgets, but this only
works for modules, not for tools, examples or other applications.
For now we'll use the Qt<Module>/Qt<Module> headers for the
modules we depend on.

Building with PCH can be disabled with -DBUILD_WITH_PCH=NO, and it only
works for versions of CMake newer than 3.15.20190829.

Change-Id: Iae52bd69acfdfd58f4cd20d3cfa3c7f42775f732
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

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