Improve ordering and naming of CGUs for non-incremental builds.

Currently there are two problems.

First, the CGUS don't end up in size order. The merging loop does sort
by size on each iteration, but we don't sort after the final merge, so
typically there is one CGU out of place. (And sometimes we don't enter
the merging loop at all, in which case they end up in random order.)

Second, we then assign names that differ only by a numeric suffix, and
then we sort them lexicographically by name, giving us an order like
this:

regex.f10ba03eb5ec7975-cgu.1
regex.f10ba03eb5ec7975-cgu.10
regex.f10ba03eb5ec7975-cgu.11
regex.f10ba03eb5ec7975-cgu.12
regex.f10ba03eb5ec7975-cgu.13
regex.f10ba03eb5ec7975-cgu.14
regex.f10ba03eb5ec7975-cgu.15
regex.f10ba03eb5ec7975-cgu.2
regex.f10ba03eb5ec7975-cgu.3
regex.f10ba03eb5ec7975-cgu.4
regex.f10ba03eb5ec7975-cgu.5
regex.f10ba03eb5ec7975-cgu.6
regex.f10ba03eb5ec7975-cgu.7
regex.f10ba03eb5ec7975-cgu.8
regex.f10ba03eb5ec7975-cgu.9

These two problems are really annoying when debugging and profiling the
CGUs.

This commit ensures CGUs are sorted by name *and* reverse sorted by
size. This involves (a) one extra sort by size operation, and (b)
padding the numeric indices with zeroes, e.g.
`regex.f10ba03eb5ec7975-cgu.01`.

(Note that none of this applies for incremental builds, where a
different hash-based CGU naming scheme is used.)

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