[clang][deps] Modules don't contribute to search path usage

To reduce the number of modules we build in explicit builds (which use strict context hash), we prune unused header search paths. This essentially merges parts of the dependency graph.

Determining whether a search path was used to discover a module (through implicit module maps) proved to be somewhat complicated. Initial support landed in D102923, while D113676 attempts to fix some bugs.

However, now that we don't use implicit module maps in explicit builds (since D120465), we don't need to consider such search paths as used anymore. Modules are no longer discovered through the header search mechanism, so we can drop such search paths (provided they are not needed for other reasons).

This patch removes whatever support for detecting such usage we had, since it's buggy and not required anymore.

Depends on D120465.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D121295