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.
MSVC_TOOLSET_VERSION for the v142 toolset.
Fixes: #19125 (closed)