[Windows build] Use "DIA SDK" in sysroot

This updates llvm/utils/sysroot.py to include the "DIA SDK" folder in the
sysroot.

It also updates the build to look for the DIA SDK there if a sysroot is set.

This requires moving LLVM_WINSYSROOT to config-ix.cmake.

For the GN build, I chose to pass a qualified path to diaguids in libs instead
of pushing a config with a `/libpath:` flag. The former requires a GN with
https://gn-review.googlesource.com/c/gn/+/12200, the latter requires D109624.
The former is more like the cmake build, arguably a bit simpler, and it's
easier to check for the wrong GN revision and easier to update GN.

Differential Revision: https://reviews.llvm.org/D109708