IRSL: Fix discovery of VS 2019 v142 toolset redistributables

Since VS 2017's v141 toolset there is no longer a simple equation to
calculate the redist name, dll version, and VS IDE version from just the
MSVC toolset version.  Refactor the logic to use hard-coded values and
warn when a new version is not supported.

Fixes: #19125
1 file changed