[sanitizers] Upgrade ios min version to 8
[sanitizers] Upgrade ios min version to 8

Differential Revision: https://reviews.llvm.org/D29787



git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@298860 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt