[gn build] Use lib_dirs instead of qualified path in libs for diaguids.lib

Follow-up to D109708: Using lib_dirs means this will work with ancient gn binaries.

Change the toolchain definitions to make lib_dirs have the right effect, and
pull out lib_switch of each of the tools while here.

This means we now do pass /LIBPATH: to link.exe, but since we invoke it directly
and not through clang-cl, this doesn't actually require D109624. And since this
is built in to GN, we don't need a config to push the flag to dependents.

This is arguably a bit more idiomatic, and it doesn't require folks to update
their GN binaries. No effective behavior change.

Differential Revision: https://reviews.llvm.org/D109763