[Driver] Remove unused #include of llvm/Support/config.h
[Driver] Remove unused #include of llvm/Support/config.h

This is a follow-up to r279112 (which removed the need for the header)
and in the same spirit as r279035 (which attempted to un-break
out-of-tree builds).

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@279348 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit d3aa353811729b890cfbf06a958f816f4cc30e7f)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang