- run xcodebuild install with the correct installation path
- change the directory that's tarred up to the one containing
/usr because we've added /System(/Library...) as a peer of /usr
We changed the cmake configuration of swift to use
CLANG_COMPILER_VERSION, but not of Clang/LLVM, so revert to
CLANG_REPOSITORY_STRING to fix the clang version.
This was causing modules created by swift to be unreadable by clang.
rdar://problem/23397407