Fix linux-header-search.cpp with CLANG_DEFAULT_CXX_STDLIB
Fix linux-header-search.cpp with CLANG_DEFAULT_CXX_STDLIB

This configuration was broken after r338294 because Clang might
be configured to always use libc++.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@338360 91177308-0d34-0410-b5e6-96231b3b80d8

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