[driver] Set the 'simulator' environment for Darwin when compiling for
iOS/tvOS/watchOS simulator

rdar://35135215

Differential Revision: https://reviews.llvm.org/D40682

llvm-svn: 320073
GitOrigin-RevId: 6cdef0efb7bf62cc44e80cd0c8973eda26d09db4
10 files changed