ELF: Only unlink regular files

[tstellar]

Summary

If the output file is a character file (e.g. /dev/null) unlinking it could
potentially destabilize the user's system. For example,
unlinking causes lld %input -o /dev/null to replace /dev/null with a
regular file, which will lead to unexpected behavior in other programs
that read from /dev/null, and worse than expected peformance for
programs that write to /dev/null.

This makes it possible to run the test-release.sh script as root.
Prior to this patch, the ELF/basic.s test would replace
/dev/null with a regular file, which would cause crashes in llvm
test-suite programs that piped /dev/null to stdin.

For example, if you run the test-relase.sh script as root,

[ruiu]

LGTM, but are you using lld as root?

[tstellar]

In D34917#797509, @ruiu wrote:

LGTM, but are you using lld as root?

Yes, the automation system I'm using uses the root.

lld/trunk/ELF/Filesystem.cpp

 // 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 binary in about 5 seconds, that waste
 // actually counts.
 //
 // This function spawns a background thread to call unlink.
 // The calling thread returns almost immediately.
 void elf::unlinkAsync(StringRef Path) {
-  if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
+  if (!Config->Threads || !sys::fs::exists(Config->OutputFile) ||
+      !sys::fs::is_regular_file(Config->OutputFile))
     return;
 
   // First, rename Path to avoid race condition. We cannot remove
   // Path from a different thread because we are now going to create
   // Path as a new file. If we do that in a different thread, the new
   // thread can remove the new file.
   SmallString<128> TempPath;
   if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))