MSVC ABI: Support C++11's auto on variables

The MSVC C++ ABI always uses the deduced type in place of auto when
generating external names for variables.

N.B.  MSVC doesn't support C++1y's 'operator auto' and this patch will
not give us said functionality.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199764 91177308-0d34-0410-b5e6-96231b3b80d8