memory leak in stage1's libsyntax after disabling vecs_implicitly_copyable

[erickt]

Stage 0's libsyntax can compile with #[deny(vecs_implicitly_copyable)] (see #5143 and #5203), but rustc has an abort in stage1. This is different from the error I mentioned in #5143. Now rustc is complaining about a memory leak. Here's the stack trace:

leaked memory in rust main loop (15 objects)
Assertion failed: (false), function ~memory_region, file /Users/erickt/rust/rust/src/rt/memory_region.cpp, line 189.

Program received signal SIGABRT, Aborted.
[Switching to process 26935 thread 0x20b]
0x00007fff8ae29212 in __pthread_kill ()
(gdb) bt
#0  0x00007fff8ae29212 in __pthread_kill ()
#1  0x00007fff8ba37af4 in pthread_kill ()
#2  0x00007fff8ba7bdce in abort ()
#3  0x00007fff8ba7ce2a in __assert_rtn ()
#4  0x000000010150d829 in memory_region::~memory_region (this=0x20b) at memory_region.cpp:189
#5  0x00000001014fcce2 in rust_sched_loop::release_task (this=0x10300d610, task=0x6) at rust_sched_loop.cpp:137
#6  0x00000001014fcd70 in rust_sched_loop::reap_dead_tasks (this=0x10300d610) at rust_task.h:204
#7  0x00000001014fd43d in rust_sched_loop::run_single_turn (this=0x10300d610) at rust_sched_loop.cpp:248
#8  0x00000001014fe665 in rust_sched_driver::start_main_loop (this=0x20b) at rust_sched_driver.cpp:50
#9  0x00000001014f8b7a in rust_thread_start (ptr=0x20b) at rust_thread.cpp:35
#10 0x00007fff8ba36742 in _pthread_start ()
#11 0x00007fff8ba23181 in thread_start ()

[graydon]

sadly lots of implicit copies have snuck back in