[AutoDiff upstream] Add flag-gated AdditiveArithmetic derivation. (#30628)

Add `AdditiveArithmetic` derived conformances for structs, gated by the
`-enable-experimential-additive-arithmetic-derivation` flag.

Structs whose stored properties all conform to `AdditiveArithmetic` can derive
`AdditiveArithmetic`:
- `static var zero: Self`
- `static func +(lhs: Self, rhs: Self) -> Self`
- `static func -(lhs: Self, rhs: Self) -> Self`
- An "effective memberwise initializer":
  - Either a synthesized memberwise initializer or a user-defined initializer
    with the same type.

Effective memberwise initializers are used only by derived conformances for
`Self`-returning protocol requirements like `AdditiveArithmetic.+`, which
require memberwise initialization.

Resolves TF-844.
Unblocks TF-845: upstream `Differentiable` derived conformances.

lib/Frontend/CompilerInvocation.cpp

@@ -442,8 +442,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_fine_grained_dependency_include_intrafile))
     Opts.FineGrainedDependenciesIncludeIntrafileOnes = true;
 
-  if (Args.hasArg(OPT_enable_experimental_differentiable_programming))
+  if (Args.hasArg(OPT_enable_experimental_additive_arithmetic_derivation))
+    Opts.EnableExperimentalAdditiveArithmeticDerivedConformances = true;
+
+  if (Args.hasArg(OPT_enable_experimental_differentiable_programming)) {
     Opts.EnableExperimentalDifferentiableProgramming = true;
+    // Differentiable programming implies `AdditiveArithmetic` derived
+    // conformances.
+    Opts.EnableExperimentalAdditiveArithmeticDerivedConformances = true;
+  }
 
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
   if (Opts.DebuggerSupport)