C++ compilation uses pretokenized headers to compile source files more quickly

This feature is only available with LLVM tool chain. It is said to be faster than precompiled headers as the `#if` haven't been evaluated so the same tokenized header can be used for when building different variant.

See https://clang.llvm.org/docs/PTHInternals.html.

No source files, tests, or entry points are named in the issue. Start by reading the linked LLVM PTHInternals documentation, then map the Gradle native compilation integration and its tests; done requires an agreed implementation scope and coverage for the supported LLVM toolchain.

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