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

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;