Add a new warning to notify users of mismatched SDK and deployment target

Summary:
This patch adds a new driver warning -Wincompatible-sdk which notifies the user when they are mismatching the version min options and the sysroot.

The patch works by checking the sysroot (if present) for an SDK name, then matching that against the target platform. In the case of a mismatch it logs a warning.

Reviewers: bob.wilson, rsmith

Subscribers: rsmith, edward-san, cfe-commits

Differential Revision: http://reviews.llvm.org/D18088

llvm-svn: 268127