cmd/vet: improve asmdecl parameter handling

The asmdecl check had hand-rolled code that
calculated the size and offset of parameters
based only on the AST.
It included a list of known named types.

This CL changes asmdecl to use go/types instead.
This allows us to easily handle named types.
It also adds support for structs, arrays,
and complex parameters.

It improves the default names given to unnamed
parameters. Previously, all anonymous arguments were
called "unnamed", and the first anonymous return
argument was called "ret".
Anonymous arguments are now called arg, arg1, arg2,
etc., depending on the index in the argument list.
Return arguments are ret, ret1, ret2.

This CL also fixes a bug in the printing of
composite data type sizes.

Updates #11041

Change-Id: I1085116a26fe6199480b680eff659eb9ab31769b
Reviewed-on: https://go-review.googlesource.com/27150
Run-TryBot: Josh Bleecher Snyder <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

Author: josharian