More modules documentation, including the straw-man import declaration syntax and "how to modularize a platform".


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177706 91177308-0d34-0410-b5e6-96231b3b80d8
Add future directions for modules

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177707 91177308-0d34-0410-b5e6-96231b3b80d8