[ARM] Use Swift error registers on non-Darwin targets

Summary:
Remove a check for `ARMSubtarget::isTargetDarwin` when determining
whether to use Swift error registers, so that Swift errors work
properly on non-Darwin ARM32 targets (specifically Android).

Before this patch, generated code would save and restores ARM register r8 at
the entry and returns of a function that throws. As r8 is used as a virtual
return value for the object being thrown, this gets overwritten by the restore,
and calling code is unable to catch the error. In turn this caused Swift code
that used `do`/`try`/`catch` to work improperly on Android ARM32 targets.

Addresses Swift bug report https://bugs.swift.org/browse/SR-5438.

Patch by John Holdsworth.

Reviewers: manmanren, rjmccall, aschwaighofer

Reviewed By: aschwaighofer

Subscribers: srhines, aschwaighofer, aemerson, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D35835

llvm-svn: 312164