Define HAVE_DIA_SDK on Windows when DIA is present.

This allows all CMake projects, as well as C++ code, to detect if
and when DIA SDK is available for use so that we can enable the
DIA-based PDB reader implementation.

Differential Revision: http://reviews.llvm.org/D7457
Reviewed By: Chandler Carruth

llvm-svn: 228669