Find SDK path more lazily in Apple Simulator platforms

Find SDK path more lazily in Apple Simulator platforms

In https://reviews.llvm.org/D122373 I delayed the search for
the SDK filepath until the simulator platform is Created.
In the qProcessInfo binary-addresses key, I have to force-Create
every platform to find one that can handle a kernel fileset;
this forced all of the simulator platforms to create, taking the
SDK filepath discovery perf hit.

This patch delays that path search further until the Apple
Simulator platform calls a method that actually needs the full
filepath; it saves the SDK name ("WatchSimulator.sdk" etc) until
it needs to expand it.

Differential Revision: https://reviews.llvm.org/D143932
rdar://103380717