Provide the correct mangling and linkage for certain unnamed nested classes.

This corrects the mangling and linkage of classes (& their member functions) in
cases like this:

  struct foo {
    struct {
      void func() { ... }
    } x;
  };

we were accidentally giving this nested unnamed struct 'no' linkage where it
should've had the linkage of the outer class. The mangling was incorrecty too,
mangling as TU-wide unnamed type mangling of $_X rather than class-scoped
mangling of UtX_.

This also fixes -Wunused-member-function which would incorrectly diagnose
'func' as unused due to it having no linkage & thus appearing to be TU-local
when in fact it might be correctly used in another TU.

Similar mangling should be applied to function local classes in similar cases
but I've deferred that for a subsequent patch.

Review/discussion by Richard Smith, John McCall, & especially Eli Friedman.

llvm-svn: 167906