Make FileOutputBuffer fail early if you pass a directory.

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

Differential Revision: https://reviews.llvm.org/D30912

llvm-svn: 297679

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

@@ -20,6 +20,9 @@
 # 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

@@ -57,6 +57,8 @@ FileOutputBuffer::create(StringRef FilePath, size_t Size, unsigned Flags) {
         // 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;