Static variant of a stubs lib is correctly tracked

cc.Module.staticVariant is used to track the corresponding static
variant of a shared variant. This change fixes a problem that the
staticVariant field is not correctly set when the lib is with stubs:
{...}. This was happening because the staticVariant was set by adding
dependency from shared variant to static variant to reuse object files.
However, for a lib with stubs, the dependency was not created because it
does not make sense to share object files for stubs lib where source
code is auto-generated.

Fixing the issue by adding dependency to the static variant with a
different dependency tag whose only purpose is to set staticVariant
field.

Bug: 122885634
Test: m (cc_test amended)
Change-Id: I7f97cbb4c4a28bf9a93839d3b91ee140835aa6af