swift234

[driver] Added support to honor the SDKROOT environment variable.

Centralized the driver-level handling of the SDK path in OutputInfo and
Driver::buildOutputInfo(), and added support for falling back to getenv(SDKROOT)
if -sdk wasn't passed (and SDKROOT is set in the environment).

Updated Swift::constructJob() to get the SDK path from the OutputInfo instead of
just passing -sdk from the original driver invocation.

Also added a driver-level check to ensure that the specified SDK is present;
if not, print out a warning. This fixes <rdar://problem/14409974>.

Added test/Driver/sdk.swift to test the -sdk and SDKROOT behavior, as well as
the nonexistent SDK warning.

Swift SVN r12815