swift234

Do all target info management in Clang, and drop -target-abi / -target-feature.

Previously we hardcoded a few important default CPUs, ABIs, and features into
Swift's driver, duplicating work in Clang. Now that we're using Clang's
driver to create the Clang "sub-compiler", we can delegate this work to Clang.

As part of this, I've dropped the options for -target-abi (which was a
frontend-only option anyway) and -target-feature (which was a hidden driver
option and is a frontend-only option in /Clang/). We can revisit this later
if it becomes interesting. I left in -target-cpu, which is now mapped
directly to Clang's -mcpu=.

Swift SVN r22449