Avoid confusing Swift 5.7 with _pointerBitWidth conditionals

#if conditional parsing is evidently broken in 5.7, and it gets confused unless things are arranged just right. Things seem to work as long as the `compiler(>=5.9)` conditional is put on its own `#if` construct on the topmost level — so let’s do that for now.

Author: lorentey

Sources/Atomics/Primitives/Primitives.native.swift.gyb

@@ -15,7 +15,13 @@
 }%
 ${autogenerated_warning()}
 
-#if ATOMICS_NATIVE_BUILTINS
+// FIXME: The conditionals below have been carefully constructed to
+// avoid confusing Swift 5.7; they can be sanitized once we drop support
+// for that version.
+#if compiler(>=5.9)
+#if !ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires native builtins on Swift 5.9")
+#endif
 import Builtin
 
 @_alwaysEmitIntoClient
@@ -39,7 +45,7 @@ internal func _atomicMemoryFence(
 % for (swiftType, builtinType, alignment) in nativePrimitives:
 
 %   if builtinType == "Int128":
-#if compiler(>=5.9) && _pointerBitWidth(_64)
+#if _pointerBitWidth(_64)
 %   end
 @usableFromInline
 @frozen
@@ -267,14 +273,18 @@ extension UnsafeMutablePointer where Pointee == ${swiftType} {
 % end
 % end
 
-#if compiler(>=5.9) && _pointerBitWidth(_64)
+#if _pointerBitWidth(_64)
 @usableFromInline internal typealias _AtomicIntStorage = _AtomicInt64Storage
 @usableFromInline internal typealias _AtomicDoubleWordStorage = _AtomicInt128Storage
-#elseif compiler(>=5.9) && _pointerBitWidth(_32)
+#elseif _pointerBitWidth(_32)
 @usableFromInline internal typealias _AtomicIntStorage = _AtomicInt32Storage
 @usableFromInline internal typealias _AtomicDoubleWordStorage = _AtomicInt64Storage
 #else
 #error("Unexpected pointer bit width")
 #endif
 
-#endif // ATOMICS_NATIVE_BUILTINS
+#else // compiler(>=5.9)
+#if ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires C shims on Swift versions below 5.9")
+#endif
+#endif // compiler(>=5.9)

Sources/Atomics/Primitives/autogenerated/Primitives.native.swift

@@ -18,7 +18,13 @@
 // #############################################################################
 
 
-#if ATOMICS_NATIVE_BUILTINS
+// FIXME: The conditionals below have been carefully constructed to
+// avoid confusing Swift 5.7; they can be sanitized once we drop support
+// for that version.
+#if compiler(>=5.9)
+#if !ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires native builtins on Swift 5.9")
+#endif
 import Builtin
 
 @_alwaysEmitIntoClient
@@ -2110,7 +2116,7 @@ extension UnsafeMutablePointer where Pointee == _AtomicInt64Storage {
   }
 }
 
-#if compiler(>=5.9) && _pointerBitWidth(_64)
+#if _pointerBitWidth(_64)
 @usableFromInline
 @frozen
 @_alignment(16)
@@ -2438,14 +2444,18 @@ extension UnsafeMutablePointer where Pointee == _AtomicInt128Storage {
 }
 #endif
 
-#if compiler(>=5.9) && _pointerBitWidth(_64)
+#if _pointerBitWidth(_64)
 @usableFromInline internal typealias _AtomicIntStorage = _AtomicInt64Storage
 @usableFromInline internal typealias _AtomicDoubleWordStorage = _AtomicInt128Storage
-#elseif compiler(>=5.9) && _pointerBitWidth(_32)
+#elseif _pointerBitWidth(_32)
 @usableFromInline internal typealias _AtomicIntStorage = _AtomicInt32Storage
 @usableFromInline internal typealias _AtomicDoubleWordStorage = _AtomicInt64Storage
 #else
 #error("Unexpected pointer bit width")
 #endif
 
-#endif // ATOMICS_NATIVE_BUILTINS
+#else // compiler(>=5.9)
+#if ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires C shims on Swift versions below 5.9")
+#endif
+#endif // compiler(>=5.9)

Sources/Atomics/Types/DoubleWord.swift

@@ -10,10 +10,17 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if ATOMICS_NATIVE_BUILTINS
+// FIXME: The conditionals below have been carefully constructed to
+// avoid confusing Swift 5.7; they can be sanitized once we drop support
+// for that version.
+#if compiler(>=5.9)
+#if !ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires native builtins on Swift 5.9")
+#endif
+
 import Builtin
 
-#if compiler(>=5.9) && _pointerBitWidth(_32)
+#if _pointerBitWidth(_32)
 @frozen
 @_alignment(8)
 public struct DoubleWord {
@@ -29,7 +36,7 @@ public struct DoubleWord {
     self.second = second
   }
 }
-#elseif compiler(>=5.9) && _pointerBitWidth(_64)
+#elseif _pointerBitWidth(_64)
 @frozen
 @_alignment(16)
 public struct DoubleWord {
@@ -63,10 +70,14 @@ extension DoubleWord {
   }
 }
 
-#else // !ATOMICS_NATIVE_BUILTINS
+#else // compiler(>=5.9)
+
+#if ATOMICS_NATIVE_BUILTINS
+#error("swift-atomics requires C shims on Swift versions below 5.9")
+#endif
 import _AtomicsShims
 public typealias DoubleWord = _AtomicsShims.DoubleWord
-#endif
+#endif // compiler(>=5.9)
 
 extension DoubleWord {
   /// Initialize a new `DoubleWord` value given its high- and