Revert "[ELF] - Avoid using memset for zero-initialization of struct member. NFC."

This reverts commit r266618. It breaks basically everything.

I think VS2013 doesn't interpret this code in the same way.
The size field (at least) is left uninitialized, causing all sorts of havok
(e.g. creating a 34GB file for a trivial hello world program).

The offending compiler reports itself as follows:

    c:\release-vs2013>cl /?
    Microsoft (R) C/C++ Optimizing Compiler Version 18.00.40629 for x64
    Copyright (C) Microsoft Corporation.  All rights reserved.

llvm-svn: 266857