[c++1z] Implement nested-namespace-definitions.

This allows 'namespace A::B { ... }' as a shorthand for 'namespace A {
namespace B { ... } }'. We already supported this correctly for error recovery;
promote that support to a full implementation.

This is not the right implementation: we do not maintain source fidelity
because we desugar the nested namespace definition in the parser. This is
tricky to avoid, since the definition genuinely does inject one named
entity per level in the namespace name.

llvm-svn: 221574