Don't use cache when building precompiled header with changed deps

Don't use cache when building precompiled header with changed deps

If one of the included files in a precompiled header is touched,
changing only its mtime, clang will produce a fatal error when the
precompiled header is then used:

    file 'foo.h' has been modified since the precompiled header 'foo.pch' was built

We need to take this into account when producing precompiled headers,
so that we don't pick out stale objects from the cache. This works
fine for direct mode, but in preprocess mode we don't have enough
information to decide if the object is still valid, so we skip the
cache entirely in that mode.