cmd/compile: use hashed symbol name for go.shape types if too long

Shape-based stenciling in the Go compiler's generic instantiation
phase looks up shape types using the underlying type of a given target
type. This has a beneficial effect in most cases (e.g. we can use the
same shape type for two different named types whose underlying type is
"int"), but causes some problems when the underlying type is a very
large structure. The link string for the underlying type of a large
imported struct can be extremely long, since the link string
essentially enumerates the full package path for every field type;
this can produce a "go.shape.struct { ... " symbol name that is
absurdly long.

This patch switches the compiler to use a hash of the underlying type
link string instead of the string itself, which should continue to
provide commoning but keep symbol name lengths reasonable for shape
types based on large imported structs.

Fixes #65030.

Change-Id: I87d602626c43172beb99c186b8ef72327b8227a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/554975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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