[llvm-symbolizer] Switch to using native symbolizer by default on Windows

llvm-symbolizer used to use the DIA SDK for symbolization on
Windows; this patch switches to using native symbolization, which was
implemented recently.

Users can still make the symbolizer use DIA by adding the `-dia` flag
in the LLVM_SYMBOLIZER_OPTS environment variable.

Differential Revision: https://reviews.llvm.org/D91814