[clang][deps] Simplify function discovering .pcm and .modulemap files

This patch simplifies (and renames) the `appendCommonModuleArguments` function.

It no longer tries to construct the command line for explicitly building modules. Instead, it only performs the DFS traversal of modular dependencies and queries the callbacks to collect paths to `.pcm` and `.modulemap` files.

This makes it more flexible and usable in two contexts:

* Generating additional command line arguments for the main TU in modular build. The `std::vector<std::string>` output parameters can be used to manually generate appropriate command line flags.
* Generate full command line for a module. The output parameters can be the corresponding parts of `CompilerInvocation`. (In a follow-up patch.)

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D100531