Refactor Darwin driver to refer to runtimes by component

Summary:
In r335809, Petr Hosek lays out support for what he calls the multiarch
runtimes layout. This new way of laying out the directories for runtime
libraries is workable for all platforms. Petr did some of the common
infrastructure and made it work for Linux and Fuscia. This patch is a
cleanup to the Darwin and MachO drivers to serve as a step toward
supporting it in Darwin.

This patch does primarily two things:
(1) Changes the APIs for how the Darwin driver refers to compiler-rt
libraries to use the component names, similar to how Linux and Fuscia do

(2) Removes some legacy functionality for supporting macOS versions
before 10.6. This functionality is effectively dead code because in
r339277, the support was removed from compiler-rt for generating the 10.4
runtime support library, and Xcode 10 (currently in beta) removes
libgcc_s.10.4 and libgcc_s.10.5 from the macOS SDK.

With this patch landed a subsequent patch can modify
MachO::AddLinkRuntimeLib to support the multiarch runtimes layout.

Worth noting: None of the removed functionality was actually covered in
the test suite. So no test case updates are required.

Reviewers: phosek, bruno, arphaman

Reviewed By: phosek, arphaman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50618

llvm-svn: 339807

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