Namespaces handling improvements. Share methods between the same name scopes. Do not lookup upper scope for qualified name.

[TIMONz1535]

I have no idea what I'm doing, I have paws, but I fixed it.

• Enable allowDuplicates for namespace symbols, so you can declare multiple namespaces with the same name. No more "redefinition" error.
• Reuse the NamespaceBinder of the first namespace with the current name, so you can share methods between all scopes.
• When lookup namespace container use the Last one, because it has all members. So don't report "ambiguous" error in that case.
• Don't extrapolate function lookup to the upper scope for a qualified name, like invoke Shading::Decals::foo but get Shading::foo. Actually we need to check members in the container only, not in the whole binder tree.
• Lots of tests for the namespace behavior. Correlates with the latest DXC release.

Known issues:
I didn't touch anything with struct, enum scopes. Probably it has similar issues.
Don't use nested namespaces with the same name, like namespace Shading { namespace Shading {} } it doesn't work properly.

example

namespace Shading { void foo(int x) {} }
namespace Shading { void foo() {} }
namespace Culling { namespace Decals { void foo(int x) {} } }
namespace Culling { namespace Decals { void foo() {} } }