[clang][DependencyScanner] Include the working directory in the context hash (#73719)

The working directory is included in the PCM, but is not currently part
of the context hash. This causes problems because different builds of a
PCM with exactly the same command line can end up with different binary
content for a PCM. If a build system tracks tasks by both working
directory and command line, it may build a given PCM multiple times,
causing a "module file out of date" error when loading the PCM due to
different sizes.