debug/dwarf: more graceful handling of unsupported types

Enhance the type decoder to do a better job handling unknown type
tags. DWARF has a number of type DIEs that this package doesn't handle
(things like "pointer to member" types in C++); avoid crashing for
such types, but instead return a placeholder "UnsupportedType" object
(this idea suggested by Austin). This provides a compromise between
implementing the entire kitchen sink and simply returning an error
outright on any unknown type DIE.

Fixes #29601.

Change-Id: I2eeffa094c86ef3a2c358ee42e8e629d74cec2ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/158797


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>