[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.
Fix a corner case with the handling of noreturn functions.
If a noreturn function was the last function in a section,
we wouldn't correctly back up the saved-pc value into the
correct section leading to us showing the wrong function in
the backtrace.

Also add a backtrace test with an attempt to elicit this
particular layout.  It happens to work out with clang -Os
but other compilers may not quite get the same layout I'm
getting at that opt setting.  We'll still be exercising the
basic noreturn handling in the unwinder even if we don't get
one function at the very end of a section.

<rdar://problem/16051613>