Enforce module decl-use restrictions and private header restrictions in textual headers

Per the documentation, these restrictions were intended to apply to textual headers but previously this didn't work because we decided there was no requesting module when the `#include` was in a textual header.

A `-cc1` flag is provided to restore the old behavior for transitionary purposes.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D132779