Add 1st draft of copy elision

[mppf]

This PR adds a 1st draft of copy elision. It works during callDestructors
and looks for a copy being the last mention of a variable before the
deinitializer is run for that variable. In that event, it elides the copy
and does not deinitialize the variable.

Copy elision is discussed in issues #13704, #9490, #9421,
#14860, #14870, and #14874.

This PR is closest to the proposal in #14860 of any of those,
but it does not quite match that proposal.

In the future, we expect to separate the effort of deciding which copies
are elide-able from the work of doing the actual elision. Since the
elision involves not deinitializing something, it makes sense to continue
to do that in callDestructors. However, the work of deciding which copies
are elide-able will move to some part of the compiler before nil checking
(because nil checking can find certain errors after an elided copy makes
a variable invalid). Additionally we are planning to use a simpler rule
for when a copy can be elided (#14874).

Reviewed by @benharsh - thanks!

• full local testing
• full local gasnet testing

[benharsh]

There are some commented-out lines of code in the compiler to clean up. Otherwise looks good.