[DebugInfo] Emit ObjC methods as part of interface

[DebugInfo] Emit ObjC methods as part of interface

As brought up during the discussion of the DWARF5 accelerator tables,
there is currently no way to associate Objective-C methods with the
interface they belong to, other than the .apple_objc accelerator table.

After due consideration we came to the conclusion that it makes more
sense to follow Pavel's suggestion of just emitting this information in
the .debug_info section. One concern was that categories were
emitted in the .apple_names as well, but it turns out that LLDB doesn't
rely on the accelerator tables for this information.

This patch changes the codegen behavior to emit subprograms for
structure types, like we do for C++. This will result in the
DW_TAG_subprogram being nested as a child under its
DW_TAG_structure_type. This behavior is only enabled for DWARF5 and
later, so we can have a unique code path in LLDB with regards to
obtaining the class methods.

This was tested on the LLDB side and doesn't lead to a regression.
There's already code in place to deal with member functions in C++,
which deals with this transparently.

For more background please refer to the discussion on the mailing list:
http://lists.llvm.org/pipermail/llvm-dev/2018-June/123986.html

Differential revision: https://reviews.llvm.org/D48241

llvm-svn: 335757