[Driver] Refactor ToolChains into separate files.

[troughton]

This PR refactors ToolChains.cpp into four separate files: the platform-specific UnixToolChains.cpp, DarwinToolChains.cpp, and WindowsToolChains.cpp, along with keeping the general-purpose ToolChains.cpp.

It also factors together some shared functionality, and I've applied clang-format to all affected files since the formatting was previously inconsistent.

Note that this PR does require moving away from free functions in ToolChains.cpp; I've instead added them to the ToolChain base class.

No functionality change intended.

@jrose-apple As discussed in #13236

[compnerd]

Nice, thanks for doing this!

[jrose-apple]

Definite improvement. Thanks, Thomas!

[jrose-apple]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 9f20a6832caa6ee3b10e628576c4c438fa6c3cf0

[troughton]

Looks like CI hasn't picked up the changes to the CMakeLists file.

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 9f20a6832caa6ee3b10e628576c4c438fa6c3cf0

[jrose-apple]

No, the case is wrong! *ToolChains.cpp vs. *Toolchains.cpp.

[troughton]

My mistake - but wow, that's a trap. The case was actually right on my local filesystem, but because I was working on macOS, git config defaults to being case insensitive (core.ignorecase). My first commit had the wrong case in the naming for the *ToolChains.cpp files, and when I fixed it and amended the commit Git didn't pick up the change.

[jrose-apple]

That's what PR testing is for! I've done it too, don't worry.

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 9f20a6832caa6ee3b10e628576c4c438fa6c3cf0

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 9f20a6832caa6ee3b10e628576c4c438fa6c3cf0

[troughton]

@jrose-apple Could you please trigger CI testing again?

That time Git added the files rather than recognising them as a rename, and my local checkout was convinced there was only one copy. I had to use the web interface to actually delete the lowercased files.

[gottesmm]

@swift-ci Please test

[gottesmm]

@swift-ci Please test