Switch NamespaceDecl from its own hand-rolled redeclaration chain over

Switch NamespaceDecl from its own hand-rolled redeclaration chain over
to Redeclarable<NamespaceDecl>, so that we benefit from the improveed
redeclaration deserialization and merging logic provided by
Redeclarable<T>. Otherwise, no functionality change.

As a drive-by fix, collapse the "inline" bit into the low bit of the
original namespace/anonymous namespace, saving 8 bytes per
NamespaceDecl on x86_64.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147729 91177308-0d34-0410-b5e6-96231b3b80d8