On reading DeclContexts from PCH, check for visible updates even if the context was...

On reading DeclContexts from PCH, check for visible updates even if the context was empty in the original version. Also, if there are any, tell the context that it has external visible decls. This fixes the problem that a namespace that was empty in the initial PCH (could also happen if the initial PCH didn't include any std header but caused implicit creation of namespace std, e.g. due to implicit declaration of a virtual destructor) never found any declaration declared in *any* chained PCH. Very ugly when the chained PCH includes all that std stuff, as the errors were effectively the same as not including std headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130102 91177308-0d34-0410-b5e6-96231b3b80d8