Remove existing file in a separate thread asynchronously.

On Linux (and probably on other Unix-like systems), unlink(2) is
noticeably slow. It takes 250 milliseconds to remove a 1 GB file
on ext4 filesystem on my machine, whether the file is on SSD or
on a spinning disk.

To create a new result file, we remove existing file first. So, if
you repeatedly link a 1 GB program in a regular compile-link-debug
cycle, every cycle wastes 250 milliseconds only to remove a file.

Since LLD can link a 1 GB in about 5 seconds, that waste actually
matters.

This patch defines `unlinkAsync` function. The function spawns a
background thread to call unlink. The calling thread returns
almost immediately.

Differential Revision: https://reviews.llvm.org/D27295

llvm-svn: 288680