stdlib: make sure that SetAlgebra.init(sequence) is on the fast path.

In order to fully optimize OptionSet literals, it's important that this function is inlined and fully optimized.
So far this was done by chance, but with COW representation it needs a hint to the optimizer.

Author: eeckstein

stdlib/public/core/SetAlgebra.swift

@@ -409,6 +409,8 @@ extension SetAlgebra {
   public init<S: Sequence>(_ sequence: __owned S)
     where S.Element == Element {
     self.init()
+    // Needed to fully optimize OptionSet literals.
+    _onFastPath()
     for e in sequence { insert(e) }
   }
 

test/SILOptimizer/optionset.swift

@@ -14,6 +14,7 @@ public struct TestOptions: OptionSet {
 
 // CHECK:      sil @{{.*}}returnTestOptions{{.*}}
 // CHECK-NEXT: bb0:
+// CHECK-NEXT:   builtin
 // CHECK-NEXT:   integer_literal {{.*}}, 15
 // CHECK-NEXT:   struct $Int
 // CHECK-NEXT:   struct $TestOptions
@@ -22,18 +23,19 @@ public func returnTestOptions() -> TestOptions {
     return [.first, .second, .third, .fourth]
 }
 
-// CHECK:      sil @{{.*}}returnEmptyTestOptions{{.*}}
-// CHECK-NEXT: bb0:
-// CHECK-NEXT:   integer_literal {{.*}}, 0
-// CHECK-NEXT:   struct $Int
-// CHECK-NEXT:   struct $TestOptions
-// CHECK-NEXT:   return
+// CHECK: sil @{{.*}}returnEmptyTestOptions{{.*}}
+// CHECK:   [[ZERO:%[0-9]+]] = integer_literal {{.*}}, 0
+// CHECK:   [[ZEROINT:%[0-9]+]] = struct $Int ([[ZERO]]
+// CHECK:   [[TO:%[0-9]+]] = struct $TestOptions ([[ZEROINT]]
+// CHECK:   return [[TO]]
+// CHECK: } // end sil function {{.*}}returnEmptyTestOptions{{.*}}
 public func returnEmptyTestOptions() -> TestOptions {
     return []
 }
 
 // CHECK:        alloc_global @{{.*}}globalTestOptions{{.*}}
 // CHECK-NEXT:   global_addr
+// CHECK-NEXT:   builtin
 // CHECK-NEXT:   integer_literal {{.*}}, 15
 // CHECK-NEXT:   struct $Int
 // CHECK-NEXT:   struct $TestOptions