[clang][deps] Give the fake file a unique name in by-module-name scans

When scanning dependencies of a module, the command line we're given doesn't have an input file, which the driver needs to be happy. We've been creating a fake in-memory input file named after the module. However, this can hide files/directories on the actual filesystem, leading to errors.

This patch works around that issue by generating a unique file name, which won't collide with the actual file system.

We could also change the driver APIs so that we're able to specify an "assumed" input file. This would be more work, though, since the driver assumes the input name comes from the actual command-line.

Depends on D140176.

Reviewed By: artemcm

Differential Revision: https://reviews.llvm.org/D140177