[docs] Document the one-phase style compilation to c++ standard modules

[ChuanqiXu]

Summary

This is the followup of D134267. The patch documents the new behavior with D134267.

[avogelsgesang]

clang/docs/StandardCPlusPlusModules.rst

916

936

[ChuanqiXu]

Address comments.

[ChuanqiXu]

Refine the behavior about partitions.

[tschuett]

Thanks for doing this! The traditional Clang Header modules call this implicit and explicit modules.

Feel free to ignore, but I found this awesome article about implicit and explicit modules in Swift. They have same nomenclature and issues.

https://forums.swift.org/t/explicit-module-builds-the-new-swift-driver-and-swiftpm/36990

[ChuanqiXu]

In D134269#3813568, @tschuett wrote:

Thanks for doing this! The traditional Clang Header modules call this implicit and explicit modules.

Feel free to ignore, but I found this awesome article about implicit and explicit modules in Swift. They have same nomenclature and issues.

https://forums.swift.org/t/explicit-module-builds-the-new-swift-driver-and-swiftpm/36990

Thanks! If you have other opinions about the interface, you can discuss it in https://reviews.llvm.org/D134267.

[ChuanqiXu]

Update to reflect the change in D134267

[tahonermann]

clang/docs/StandardCPlusPlusModules.rst

228–229

When an importable module unit is compiled to an object file,
a corresponding BMI is implicitly generated.

233–234

• (1) If a file name is specified with the -fmodule-bmi-output={BMI-destination-path} option, then the BMI will be written to {BMI-destination-path}.

235–237

What happens if the source file name does not have an extension? E.g., for clang -c -std=c++20 -xc++-module file?

235–239

• (2) Otherwise, the BMI will be written to a file within the module cache with a name constructed from the source file name with a .pcm extension. For example, if the source file is M.cppm, then the BMI will be implicitly written to a file named M.pcm in the module cache. See :ref:Module Cache Path<Module Cache Path> for the rules about the module cache.

[ChuanqiXu]

Suspended due to the dependent patches suspended.