Rather than hard-coding a set of locations to check, use vswhere (which
is installed by VS 2017 and later), and construct the vcvars64.bat path
relative to that. This will allow Windows builds to work without special
configuration for people who have VS installed at a custom path.

Also adds error logging with different messages for each failure point,
so that rather than the not-very-informative 'failed to find
vcvars64.bat' message, the failure will provide feedback about what to
do.

This is an interim solution; later this will be replaced by a
VisualStudio class with associated validator to match the structure of
the other toolchains.

Fixes #33249