cmd/compile: make alias decls work correctly under all circumstances

[griesemer]

[edit: This is now the umbrella issue for work on alias declarations. This includes parameterized alias declarations; see issue #46477. It also includes proper names for aliases in error messages.]

The original fix for #18640 was only partially correct. However, removing that incorrect code leads to another cycle related issue for the test case:

type (
	e = f
	f = g
	g = []h
	h i
	i = j
	j = e
)

Left the original, partially correct code in place for now as it's not wrong, it just doesn't cover the expected cases and it appears to circumvent this issue.

Marking for 1.11 if we get to it but it's not a showstopper.

Commented out this test case in test/fixedbugs/issue18640.go for now.

[gopherbot]

Change https://golang.org/cl/118078 mentions this issue: cmd/compile: correct alias cycle detection

[griesemer]

Simpler reproducer:

type (
	e = []h
	h e
)

Yet, the equivalent code (after substituting e with []h) is accepted as expected:

type h []h

[griesemer]

(Slightly) simpler reproducer:

package pkg
type P = *T
type T P

The code is accepted if the order of the type declarations is swapped. The bug is likely in type-checking alias declarations that refer to defined types that have not yet been set up and which refer back to the alias type. We end up with an incorrect type T T and dowith goes belly up.