[ASan] Use dynamic shadow on 32-bit iOS and simulators

[ASan] Use dynamic shadow on 32-bit iOS and simulators

The VM layout on iOS is not stable between releases. On 64-bit iOS and
its derivatives we use a dynamic shadow offset that enables ASan to
search for a valid location for the shadow heap on process launch rather
than hardcode it.

This commit extends that approach for 32-bit iOS plus derivatives and
their simulators.

rdar://50645192
rdar://51200372
rdar://51767702

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D63586

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364105 91177308-0d34-0410-b5e6-96231b3b80d8