Repair support for launching ios/tv/watch simulator binaries through platform

and delete a bunch (but not all) redundant code. If you compare the remaining implementations of Platform*Simulator.cpp, there is still an obvious leftover cleanup task.

Specifically, this patch

- removes SDK initialization from dotest (there is equivalent but more
  complete code in Makefile.rules)

- make Platform*Simulator inherit the generic implementation of
  PlatformAppleSimulator (more can be done here)

- simplify the platform logic in Makefile.rules

- replace the custom SDK finding logic in Platform*Simulator with XcodeSDK

- adds a test for each supported simulator

Differential Revision: https://reviews.llvm.org/D81980