cmd/compile: accept string|[]byte-constrained 2nd argument in append

Similarly to what we do for the built-in function `copy`,
where we allow a string as 2nd argument to append, also
permit a type parameter constrained by string|[]byte.

While at it, change date in the manual.go2 test files so
that we don't need to constantly correct it when copying
a test case from that file into a proper test file.

Fixes #50281.

Change-Id: I23fed66736aa07bb3c481fe97313e828425ac448
Reviewed-on: https://go-review.googlesource.com/c/go/+/376214


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>