Simplify DINamespace caching in CGDebugInfo

This addresses review feedback from r302840.

By not canonicalizing namespace decls and using lexical decl context
instead of lookuing up the semantic decl context we can take advantage
of the fact that DINamespaces a reuniqued. This way non-module debug
info is unchanged and module debug info still gets distinct namespace
declarations when they ocur in different modules.

Thanks to Richard Smith for pointing this out!

llvm-svn: 302915