Make the ELFFile constructor private.

With this all clients have to use the new create method which returns
an Expected.

Fixes a crash on invalid input.

llvm-svn: 315376
diff --git a/llvm/test/Object/Inputs/invalid-buffer.elf b/llvm/test/Object/Inputs/invalid-buffer.elf
new file mode 100644
index 0000000..665d9d1
--- /dev/null
+++ b/llvm/test/Object/Inputs/invalid-buffer.elf
@@ -0,0 +1 @@
+ELF            
\ No newline at end of file