[clang] [clang][modules-driver] Add scanner to detect C++20 module presence (PR #145220)

[Michael Spencer via cfe-commits]

Bigcheese wrote:

> > This PR is part of a series aimed at implementing native support for explicit module builds from the Clang driver.
> 
> This is confusing. Since explicit modules is an unrelated term with C++20 modules.

All C++20 named modules builds are done explicitly at the moment. The terminology isn't perfect, but explicitly built means there's a call to `clang -cc1 -emit-module` and implicitly built means the compiler does it internally. This will also eventually support C++20 named modules with Clang modules acting as header units, as well as just stand alone Clang modules.

https://github.com/llvm/llvm-project/pull/145220