Just like in the spec text, use symbolic types that represent multiple
concrete types. The few signatures that get generated more than once
are only added to the symbol table once because it ignores duplicates.

BUG=angle:926

Change-Id: I216f03d22502b724dbefc87c5a021d6021c3a434
Reviewed-on: https://chromium-review.googlesource.com/251620Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Nicolas Capens <capn@chromium.org>