[COFF] Simplify ICF associated comdat handling

This is a different approach from D98993 that should achieve most of the
same benefit. The two changes are:
1. Sort the list of associated child sections by section name
2. Do not consider associated sections to have children themselves

This fixes the main issue, which was that we sometimes considered an
.xdata section to have a child .pdata section. That lead to slow links
and larger binaries (less xdata folding).

Otherwise, this should be NFC: we go back to ignoring .debug/.gljmp and
other metadata sections rather than only looking at pdata/xdata. We
discovered that we do care about other associated sections, like ASan
global registration metadata.

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