Document module.private.modulemap and module_private.map.

Requested in <rdar://problem/16188740>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205030 91177308-0d34-0410-b5e6-96231b3b80d8
Documentation: remove a spurious '1' and wrap to 80 columns


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205035 91177308-0d34-0410-b5e6-96231b3b80d8
Use constexpr again, this time portably

Responding to Justin's review of r205025.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205037 91177308-0d34-0410-b5e6-96231b3b80d8