[PCH/Modules] In ASTReader::completeVisibleDeclsMap, make sure to visit all

modules when getting the decls for a namespace or translation unit.

Otherwise the code-completion results will not be complete.

rdar://12889089

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170596 91177308-0d34-0410-b5e6-96231b3b80d8