stdlib: Make comparison functions for optional nil transparent

This will generate significantly better code if a generic optional is compared against nil.
In a generic function this ended up with a call to the == function. Now, due to mandatory inlining, it boils down to a switch_enum.

Author: eeckstein

stdlib/public/core/Optional.swift

@@ -345,6 +345,7 @@ public func ~= <T>(lhs: _OptionalNilComparisonType, rhs: T?) -> Bool {
 
 // Enable equality comparisons against the nil literal, even if the
 // element type isn't equatable
+@_transparent
 public func == <T>(lhs: T?, rhs: _OptionalNilComparisonType) -> Bool {
   switch lhs {
   case .some(_):
@@ -354,6 +355,7 @@ public func == <T>(lhs: T?, rhs: _OptionalNilComparisonType) -> Bool {
   }
 }
 
+@_transparent
 public func != <T>(lhs: T?, rhs: _OptionalNilComparisonType) -> Bool {
   switch lhs {
   case .some(_):
@@ -363,6 +365,7 @@ public func != <T>(lhs: T?, rhs: _OptionalNilComparisonType) -> Bool {
   }
 }
 
+@_transparent
 public func == <T>(lhs: _OptionalNilComparisonType, rhs: T?) -> Bool {
   switch rhs {
   case .some(_):
@@ -372,6 +375,7 @@ public func == <T>(lhs: _OptionalNilComparisonType, rhs: T?) -> Bool {
   }
 }
 
+@_transparent
 public func != <T>(lhs: _OptionalNilComparisonType, rhs: T?) -> Bool {
   switch rhs {
   case .some(_):

test/SILOptimizer/mandatory_nil_comparison_inlining.swift

@@ -0,0 +1,38 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-sil -o - -verify | FileCheck %s
+
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}generic_func
+// CHECK: switch_enum_addr
+// CHECK: return
+func generic_func<T>(x: [T]?) -> Bool {
+  return x == nil
+}
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}array_func_rhs_nil
+// CHECK: switch_enum_addr
+// CHECK: return
+func array_func_rhs_nil(x: [Int]?) -> Bool {
+  return x == nil
+}
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}array_func_lhs_nil
+// CHECK: switch_enum_addr
+// CHECK: return
+func array_func_lhs_nil(x: [Int]?) -> Bool {
+  return nil == x
+}
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}array_func_rhs_non_nil
+// CHECK: switch_enum_addr
+// CHECK: return
+func array_func_rhs_non_nil(x: [Int]?) -> Bool {
+  return x != nil
+}
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}array_func_lhs_non_nil
+// CHECK: switch_enum_addr
+// CHECK: return
+func array_func_lhs_non_nil(x: [Int]?) -> Bool {
+  return nil != x
+}
+