Wait for all threads to terminate before exitting.

I think it is not defined what would happen to detached threads
when the main thread tries to exit. That means it was not guaranteed
that unlinkAsync correctly removes a temporary file. It was also
reported that this unlinkAsync caused a crash on Windows.

This patch adds a few new functions so that the main thread always
waits for non-main threads before exitting.

I don't actually like the new two functions, runBackground and
waitForBackgroundThreads, because it looks like it is a bit
overdesigned. After all, what we are doing with these functions
is to just remove a file.

An alternative would be to do fork(2) and make the child process
remove a file asynchronously. However, it has its own problems.
Correctly forking and reclaiming a resource using waitpid(2) is not
doable unless we know our process-wide settings (such as signal mask),
but we can't make any assumption on it when lld is embedded to other
process. So I chose to stick with threads instead of multi-processes.

Differential Revision: https://reviews.llvm.org/D38571

llvm-svn: 315033