[C++20] [Modules] [NFC] Add Preprocessor methods for named modules - for ClangScanDeps (1/4)

This patch prepares the necessary interfaces in the preprocessor part
for D137527 since we need to recognize if we're in a module unit, the
module kinds and the module declaration and the module we're importing
in the preprocessor.

Differential Revision: https://reviews.llvm.org/D137526