clang-cl: Add support for /Zp

CL's /Zp flag is analogous to GCC's -fpack-struct, it controls the
default maximum alignment of records.

Differential Revision: http://reviews.llvm.org/D4671

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213958 91177308-0d34-0410-b5e6-96231b3b80d8