swift234

Use -resource-dir for iOS builds and tests instead of -I.

As part of this, have the standard library target be responsible for
symlinking Clang's headers into the resource directory, instead of the
compiler target. This makes sure the headers show up in all copies of
the build directory.

This brings our iOS testing closer to what Xcode will do, which will
hopefully avoid issues like <rdar://problem/16052579>.

Swift SVN r13890