go: syntax: parser doesn't recognize valid type parameter list

There is a (known) ambiguity for type parameter lists. For

type T[P *int] struct{}

the parser cannot tell if this is a generic type declaration or an array type declaration with length P*int. In general, people will write ~*int and there is the work-around interface{*int}; one just has to be aware of it.

But the parser also assumes that

type T[P *int, Q any] struct{}

is starting an array type declaration, yet this is clearly a valid type parameter list.

Should be fixed for 1.18 but is not a release blocker as there are work-arounds.

cc: @findleyr @ianlancetaylor