[clangd] [C++20] [Modules] Add modules suffix for 'Header' Source Switch (#131591)
Support the trivial "header"/source switch for module interfaces.
I initially thought the naming are bad and we should rename it. But
later I feel it is better to split patches as much as possible.
From the codes it looks like there are problems. e.g., `isHeaderFile`.
But let's try to fix them in different patches.