[lldb/win] Default to native PDB reader when LLVM_ENABLE_DIA_SDK=NO

Trying to use the DIA SDK reader only to fail with "DIA SDK wasn't enabled"
isn't very useful. The native PDB reader is missing some stuff, but it's still
better than nothing.

Reduces number of lldb-check-shell test failures with LLVM_ENABLE_DIA_SDK=NO
from 27 to 15.

Differential Revision: https://reviews.llvm.org/D110172