Use -Xclang to pass -internal-isystem and -internal-externc-isystem to -cc1.
Use -Xclang to pass -internal-isystem and -internal-externc-isystem to -cc1.

They are cc1 options only, so the driver was ignoring them.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@191379 91177308-0d34-0410-b5e6-96231b3b80d8

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