For Darwin builds, locate tools using xcrun when available.
For Darwin builds, locate tools using xcrun when available.

The make/platform/darwin_bni.mk file already has similar code but we
apparently neglected to add it to the clang_darwin.mk file.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@188864 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt