We need to pick up the `_swift_tsan_xxx` symbols from libswiftCore in
most cases, but sometimes we're statically linked and in that case we
want to use a local copy.
rdar://1106655213
On Darwin, `RTLD_NEXT` doesn't do what we need it to here, with the
result that if `libswiftCore`'s TSan initializer gets found first,
then `libswift_Concurrency` won't have its initializer called at all,
in spite of us using `RTLD_NEXT` to find the next definition.
Fix this by centralising the initializer in `libswiftCore` instead.
rdar://110665213
* Use the longer name ThreadSanitizer rather than TSan for the new files.
* Don't implement `tsan::consume` at all for now.
* Do the `tsan::release` for `ulock_unlock()` at the head of the function,
not at the tail.
* Add a comment to test/Sanitizers/tsan/once.swift to explain the test a
little more clearly.
rdar://110665213
