swift234

[Driver] Switch debug info to use ld's new -add_ast_path option.

Rather than embed AST info directly in binaries, we now include a special
symbol table entry that points to the serialized AST as a separate file.
This requires a very recent version of ld.

We still want to support the __SWIFT,__ast section in a binary because
that's how it's modeled in dSYM, so manually test both modes in
ASTSection_linker.swift.

Part of <rdar://problem/15796201>.

Swift SVN r20421