Don't search for sim SDK path until we know we need it

When iterating over all Platforms looking for the best one, on a Mac the
Simulator platforms (iOS, tvOS, watchOS) will first find their SDK
directory by calling xcrun, then decide if they should activate or not.
When that SDK is absent, the call to xcrun to find it can be very slow.
This patch delays that directory search until we know we're activating
this platform, so non-simulator environments don't pay a perf cost ever
time they go through the list of platforms.

Differential Revision: https://reviews.llvm.org/D122373
rdar://87960090