[modules] When we merge redecl chains or mark a decl used with an update

record, mark all subsequent decls as 'used' too, to maintain the AST invariant
that getPreviousDecl()->Used implies this->Used.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211050 91177308-0d34-0410-b5e6-96231b3b80d8