Currently, we reexport any package-scope constant, function, type, or
variable declarations needed by an inlineable function body. However,
now that we have an early pass to walk inlineable function bodies
(golang.org/cl/74110), we can simplify the logic for finding these
declarations.

The binary export format supports writing out type declarations
in-place at their first use. Also, it always writes out constants by
value, so their declarations never need to be reexported.

Notably, we attempted this before (golang.org/cl/36170) and had to
revert it (golang.org/cl/45911). However, this was because while
writing out inline bodies, we could discover variable/function
dependencies. By collecting variable/function dependencies during
inlineable function discovery, we avoid this problem.

While here, get rid of isInlineable. We already typecheck inlineable
function bodies during inlFlood, so it's become a no-op. Just move the
comment explaining parameter numbering to its caller.

Change-Id: Ibbfaafce793733675d3a2ad98791758583055666
Reviewed-on: https://go-review.googlesource.com/103864Reviewed-by: Robert Griesemer <gri@golang.org>