Emit used/dllexport inline method definitions in nested classes (PR19743, PR11170)

The previous code that was supposed to handle this didn't work
since parsing of inline method definitions is delayed to the end
of the outer class definition. Thus, when HandleTagDeclDefinition()
got called for the inner class, the inline functions in that class
had not been parsed yet.

Richard suggested that the way to do this is by handling inline
method definitions through a new ASTConsumer callback.

I really wanted to call ASTContext::DeclMustBeEmitted() instead of
checking for attributes, but doing that causes us to compute linkage,
and then we fail with "error: unsupported: typedef changes linkage
of anonymous type, but linkage was already computed" on tests like
this: (from SemaCXX/undefined-internal.cpp) :-/

  namespace test7 {
    typedef struct {
      void bar();
      void foo() { bar(); }
    } A;
  }

Differential Revision: http://reviews.llvm.org/D3809