<rdar://problem/13477190> Give the Clang module cache directory some structure, so it's easier to find.

We now put the Clang module cache in
<system-temp-directory>/org.llvm.clang/ModuleCache. Perhaps some day
there will be other caches under <system-temp-directory>/org.llvm.clang>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177671 91177308-0d34-0410-b5e6-96231b3b80d8