Make sure flatten globals doesn't insert uses of globals that previously had no uses.

This was the deeper cause of the issues I fixed in https://codereview.chromium.org/33233002/ and is therefore a better fix. (The problem was that StripDeadPrototypes was not stripping a variable that was actually dead because the bitcast constexpr inserted by FlattenGlobals referred to it.) I reverted most of that CL's changes, though not the comment and test cleanup.

R=dschuff@chromium.org
TEST= pnacl-clang++ pnacl/git/libcxx/test/input.output/iostream.objects/narrow.stream.objects/clog.pass.cpp -stdlib=libc++

Review URL: https://codereview.chromium.org/34843003

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