Inline-always floating-point .radix and additive arithmetic .zero for 5.1 (#25799)

* Make AdditiveArithmetic.zero @_transparent (#25658)

* FloatingPoint.radix should be inlinable.

* Switch transparent to inlinable+inline(__always).

Author: stephentyrone

stdlib/public/core/FloatingPoint.swift

@@ -2048,6 +2048,7 @@ extension BinaryFloatingPoint {
   /// following formula, where `**` is exponentiation:
   ///
   ///     let magnitude = x.significand * F.radix ** x.exponent
+  @inlinable @inline(__always)
   public static var radix: Int { return 2 }
 
   /// Creates a new floating-point value using the sign of one value and the

stdlib/public/core/Integers.swift

@@ -133,6 +133,7 @@ public extension AdditiveArithmetic where Self : ExpressibleByIntegerLiteral {
   ///
   /// Zero is the identity element for addition. For any value,
   /// `x + .zero == x` and `.zero + x == x`.
+  @inlinable @inline(__always)
   static var zero: Self {
     return 0
   }