Make FileOutputBuffer fail early if you pass a directory.

[ruiu]

Summary

Previously, it created a temporary directory and then fail when
FileOutputBuffer renames that file to the destination file (which is
actually a directory name).

lld/test/ELF/early-exit-for-bad-paths.s

 
 # NO-DIR: error: cannot open output file does_not_exist/output:
 # DIR-IS-FILE: error: cannot open output file {{.*}}/dir_is_a_file:
 
 # We should exit before doing the actual link. If an undefined symbol error is
 # discovered we haven't bailed out early as expected.
 # CHECK-NOT: undefined_symbol
 
+# RUN: not ld.lld %t.o -o / 2>&1 | FileCheck %s -check-prefixes=ROOT
+# ROOT: error: cannot open output file /
+
   .globl _start
 _start:
   call undefined_symbol

llvm/lib/Support/FileOutputBuffer.cpp

case sys::fs::file_type::file_not_found:
       // If file does not exist, we'll create one.
       break;
     case sys::fs::file_type::regular_file: {
         // If file is not currently writable, error out.
         // FIXME: There is no sys::fs:: api for checking this.
         // FIXME: In posix, you use the access() call to check this.
       }
       break;
+    case sys::fs::file_type::directory_file:
+      return errc::is_a_directory;
     default:
       if (EC)
         return EC;
       IsRegular = false;
   }
 
   if (IsRegular) {
     // Delete target file.