[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.

llvm-svn: 313316