[Toolchains] Cygwin toolchain inherits from Unix

There is currently a great deal of duplication across the
`GenericUnix` and `Windows` toolchains. The Android port will
add even more duplication.

To mitigate this, have `Windows` inherit from `GenericUnix`, and
have them share most of their implementation.

In addition, rename `Windows` to `Cygwin` (it would be pretty strange
to have a `Windows` toolchain inherit from something named `*Unix`).

lib/Driver/Driver.cpp

@@ -2035,7 +2035,7 @@ const ToolChain *Driver::getToolChain(const ArgList &Args) const {
       TC = new toolchains::GenericUnix(*this, Target);
       break;
     case llvm::Triple::Win32:
-      TC = new toolchains::Windows(*this, Target);
+      TC = new toolchains::Cygwin(*this, Target);
       break;
     default:
       TC = nullptr;