MSVC: support version preference with search

Extend the logic for the WinSDK and UCRT handling to prefer a user
specified version of the VisualC++ tools and Windows SDK.  This allows
us to now perform the regular search for the installation but select the
exact version of the SDK or VC++ tools to override the latest version.
Similar to the other flags controlling this behaviour, if the user
specifies a value, we will not perform validation on the input and will
attempt to prefer that, particularly in the case of VisualC++ tools
where no fallback occurs.

Reviewed by: hans
Differential Revision: https://reviews.llvm.org/D145517