[dev.link] cmd/link: don't always load type symbols
[dev.link] cmd/link: don't always load type symbols

The type symbols were always loaded in LoadFull, as the old
sym.Symbol-based DWARF generation needs them to present. Now we
have converted the DWARF type generation pass, which uses the
loader symbols directly. No need to load the type symbols if they
are not live.

This reduces loaded symbols by ~18% for cmd/compile.

Change-Id: Ic1f6fcdff845f3c62784d05ad716b1742f249481
Reviewed-on: https://go-review.googlesource.com/c/go/+/222018
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,nawawi/go,golang/go,akutz/go,golang/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,akutz/go