[clang][modules] NFCI: Distinguish as-written and effective umbrella directories

For modules with umbrellas, we track how they were written in the module map. Unfortunately, the getter for the umbrella directory conflates the "as written" directory and the "effective" directory (either the written one or the parent of the written umbrella header).

This patch makes the distinction between "as written" and "effective" umbrella directories clearer. No functional change intended.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151581