VS: Allow toolset version selection to specify default toolset

Teach the feature added by commit v3.12.0-rc1~38^2 (VS: Add option to
select the version of the toolset used by VS 2017, 2018-05-19) to accept
the default toolset version in addition to older versions.  If the
default toolset version is supplied, simply clear it so the default will
be used.

Fixes: #18107
Status Job ID Name Coverage
  External
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