Fix up default header paths for CloudABI.
Fix up default header paths for CloudABI.

CloudABI is a pure cross compilation target. This means that we should
not add /usr/include and /usr/local/include. Instead, headers are stored
in $sysroot/$triple/include.

The method of going back to the sysroot (by using "../../..") is also
used in this function for some of the other environments (e.g., MinGW).


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@231913 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang