Previously, for slow map key types (i.e., any type other than a 32-bit
or 64-bit plain memory type), we would rewrite

    defer delete(m, k)

into

    ktmp := k
    defer delete(m, &ktmp)

However, if the defer statement was inside a loop, we would end up
reusing the same ktmp value for all of the deferred deletes.

We already rewrite

    defer print(x, y, z)

into

    defer func(a1, a2, a3) {
        print(a1, a2, a3)
    }(x, y, z)

This CL generalizes this rewrite to also apply for slow map deletes.

This could be extended to apply even more generally to other builtins,
but as discussed on #24259, there are cases where we must *not* do
this (e.g., "defer recover()"). However, if we elect to do this more
generally, this CL should still make that easier.

Lastly, while here, fix a few isues in wrapCall (nee walkprintfunc):

1) lookupN appends the generation number to the symbol anyway, so "%d"
was being literally included in the generated function names.

2) walkstmt will be called when the function is compiled later anyway,
so no need to do it now.

Fixes #24259.

Change-Id: I70286867c64c69c18e9552f69e3f4154a0fc8b04
Reviewed-on: https://go-review.googlesource.com/99017
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>