On OS X, use xcrun (if present) to find the clang to use for static analysis if no clang can be found relative to the location of scan-build.
On OS X, use xcrun (if present) to find the clang to use for static analysis if
no clang can be found relative to the location of scan-build.

Fixes <rdar://problem/11691794>

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@162535 91177308-0d34-0410-b5e6-96231b3b80d8

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