Disable compilation of llvm-pdbdump for versions of MSVC < 2013.

Certain aspects of llvm-pdbdump require language support only present in
MSVC 2013 and higher.  Since this is strictly a utility, and since we hope
to drop support for MSVC 2012 soon, don't build this unless MSVC 2013 or
higher.

llvm-svn: 227479