[modules] Simplify -cc1 interface for enabling implicit module maps.

We used to have a flag to enable module maps, and two more flags to enable
implicit module maps. This is all redundant; we don't need any flag for
enabling module maps in the abstract, and we don't usually have -fno- flags for
-cc1. We now have just a single flag, -fimplicit-module-maps, that enables
implicitly searching the file system for module map files and loading them.

The driver interface is unchanged for now. We should probably rename
-fmodule-maps to -fimplicit-module-maps at some point.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239789 91177308-0d34-0410-b5e6-96231b3b80d8