[AutoDiff upstream] Deduplicate floating-point Differentiable conformances.

Move `Differentiable` conformances out of Differentiable.swift.
All `Differentiable` conformances exist in separate files.

Author: dan-zheng

stdlib/public/Differentiation/Differentiable.swift

@@ -44,19 +44,3 @@ public extension Differentiable where TangentVector == Self {
     self += direction
   }
 }
-
-//===----------------------------------------------------------------------===//
-// `Differentiable` conformances
-//===----------------------------------------------------------------------===//
-
-extension Float: Differentiable {
-  public typealias TangentVector = Self
-}
-extension Double: Differentiable {
-  public typealias TangentVector = Self
-}
-#if (arch(i386) || arch(x86_64)) && !(os(Windows) || os(Android))
-extension Float80: Differentiable {
-  public typealias TangentVector = Self
-}
-#endif

stdlib/public/Differentiation/FloatingPointDifferentiation.swift.gyb

@@ -44,6 +44,10 @@ extension ${Self}: Differentiable {
   }
 }
 
+//===----------------------------------------------------------------------===//
+// Derivatives
+//===----------------------------------------------------------------------===//
+
 /// Derivatives of standard unary operators.
 ${Availability(bits)}
 extension ${Self} {