[C++20] [Modules] Disambuguous Clang module and C++20 Named module further

This patch tries to make the boundary of clang module and C++20 named
module more clear.

The changes included:

- Rename `TranslationUnitKind::TU_Module` to
  `TranslationUnitKind::TU_ClangModule`.
- Rename `Sema::ActOnModuleInclude` to `Sema::ActOnAnnotModuleInclude`.
- Rename `ActOnModuleBegin` to `Sema::ActOnAnnotModuleBegin`.
- Rename `Sema::ActOnModuleEnd` to `Sema::ActOnAnnotModuleEnd`.
- Removes a warning if we're trying to compile a non-module unit as
  C++20 module unit. This is not actually useful and makes (the future)
  implementation unnecessarily complex.

This patch meant to be a NFC fix. But it shows that it fixed a bug
suprisingly that previously we would surppress the unused-value warning
in named modules. Because it shares the same logic with clang modules,
which has headers semantics. This shows the change is meaningful.