VS: Teach generators how to mark per-config source files

Add internal infrastructure for looping over all sources for all
configurations and generating each source with exclusion marks
for configurations in which they do not participate.  This does
not yet make per-config sources available in general but does
set up some of the needed infrastructure.

Unfortunately doing this cleanly will require major refactoring of both
the VS 7-9 generators and the VS 10+ generators (for separate reasons).
Instead add some extra internal structures to carry information where we
need it.

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