-gmodules: Emit debug info for implicit module imports via #include.

When a type is only used as a template parameter and that type is the
only type imported from another #include'd module, no skeleton CU for
that module is generated, so a consumer doesn't know where to find the
type definition. By emitting an import declaration, we can force a
skeleton CU to be generated for each imported module.

rdar://problem/36266156

llvm-svn: 321754