Revert "Roll forward r314928"

This appears to be miscompiling Clang, as shown on two Windows bootstrap
bots:
http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015/builds/7611
http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/6870

Nothing else is in the blame list. Both emit errors on this valid code
in the Windows ucrt headers:

C:\...\ucrt\malloc.h:95:32: error: invalid operands to binary expression ('char *' and 'int')
            _Ptr = (char*)_Ptr + _ALLOCA_S_MARKER_SIZE;
                   ~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~

I am attempting to reproduce this now.

This reverts r315044

llvm-svn: 315108