Clang Driver: Use Apple ld64's new @response-file support.

In XCode 12, ld64 got support for @files, in addition to the old
-filelist mechanism. Response files allow passing all command-line
arguments to the linker via a file, rather than just filenames, and is
therefore preferred.

Because of the way response-file support is currently implemented as
part of the Tool class in Clang, this change requires an ugly backdoor
function to access Args. A follow-up commit fixes this, but I've
ordered this change first, for easier backportability.

I've added no tests here, because unfortunately, there don't appear to
be _any_ response-file emission automated tests, and I don't see an
obvious way to add them. I've tested that this change works as
expected locally.

Differential Revision: https://reviews.llvm.org/D82777

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