[modules] Fix bug where an anonymous namespace could cause the containing

[modules] Fix bug where an anonymous namespace could cause the containing
namespace to not merge properly.

We have an invariant here: after a declaration reads its canonical declaration,
it can assume the canonical declaration is fully merged. This invariant can be
violated if deserializing some declaration triggers the deserialization of a
later declaration, because that later declaration can in turn deserialize a
redeclaration of that first declaration before it is fully merged.

The anonymous namespace for a namespace gets stored with the first declaration
of that namespace, which may be before its parent namespace, so defer loading
it until after we've finished merging the surrounding namespace.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232455 91177308-0d34-0410-b5e6-96231b3b80d8