[clang] When loading preamble from AST file, re-export modules in Sema.

This addresses a FIXME in ASTReader.

Modules were already re-exported for Preprocessor, but not for Sema.
The result was that, with -fmodules-local-submodule-visibility, all AST
nodes belonging to a module that was loaded in a premable where not
accesible from the main part of the file and a diagnostic recommending
importing those modules would be generated.

Differential Revision: https://reviews.llvm.org/D86069