[libcxx] Refactoring target_info.py

This patch makes it easier to support running the lit tests for new and
unusual platforms. It will break existing users that set
LIBCXX_TARGET_INFO to anything other than the default. I think this is
fine, because the old LIBCXX_TARGET_INFO wasn't terribly useful.

The old way of supporting the different test platforms was to have
conditional code scattered throughout config.py. New platforms would need
to add conditionals there. Alternatively, the new platform could set
no_default_flags to true, and reconstitue almost the entire compile and
link line, including things that don't vary across platforms.

The new way of supporting new platforms is to create a new target info
class, and have make_target_info return an instance of it. For platforms
supported in-tree, that will be done by modifying make_target_info. For
out-of-tree platforms, users can set LIBCXX_TARGET_INFO at cmake configure
time.

The target info sub-classes can provide fine-grained information back to
config.py. The hooks that will most commonly be provided will be
add_cxx_compile_flags and add_cxx_link_flags. These hooks can provide the
platform specific flags, while letting config.py handle all the invariant
flags.

Target info hooks were added for each area that the existing config.py had
platform specific behavior. config.py is now mostly free of platform
specific conditionals.

This patch was tested on Linux x86_64. I both targeted Linux x86_64, and
an out-of-tree platform with a custom target_info. In both cases I was
able to run libcxx and libcxxabi tests. I do not have access to FreeBSD,
Darwin, or Windows machines that are set up for lit testing.

llvm-svn: 256588