Implement -iwithsysroot, an apple extension which is a close cousin of -isystem.

Instead of implementing -isystem, I accidentally implemented this cousin.  Next
up is to implement -isystem right.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111966 91177308-0d34-0410-b5e6-96231b3b80d8