[Module map] Introduce a private module re-export directive.

[Module map] Introduce a private module re-export directive.

Introduce a new "export_as" directive for top-level modules, which
indicates that the current module is a "private" module whose symbols
will eventually be exported through the named "public" module. This is
in support of a common pattern in the Darwin ecosystem where a single
public framework is constructed of several private frameworks, with
(currently) header duplication and some support from the linker.

Addresses rdar://problem/34438420.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@313316 91177308-0d34-0410-b5e6-96231b3b80d8