[CMake] Use libc++ and compiler-rt for sanitizers
[CMake] Use libc++ and compiler-rt for sanitizers

When building runtimes for Linux as part of Fuchsia toolchain, use
libc++ and compiler-rt for sanitizers.

Differential Revision: https://reviews.llvm.org/D49331

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@337117 91177308-0d34-0410-b5e6-96231b3b80d8

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