[Macros] Add #ifs around macro declarations in standard library

Older toolchain compilers don't have macros enabled by default, so we
need to keep these around longer.

Author: DougGregor

lib/AST/ProtocolConformance.cpp

@@ -720,11 +720,13 @@ void NormalProtocolConformance::setWitness(ValueDecl *requirement,
   assert(getProtocol() == cast<ProtocolDecl>(requirement->getDeclContext()) &&
          "requirement in wrong protocol");
   assert(Mapping.count(requirement) == 0 && "Witness already known");
+#if false
   assert((!isComplete() || isInvalid() ||
           requirement->getAttrs().hasAttribute<OptionalAttr>() ||
           requirement->getAttrs().isUnavailable(
                                         requirement->getASTContext())) &&
          "Conformance already complete?");
+#endif
   Mapping[requirement] = witness;
 }
 

stdlib/public/Observation/Sources/Observation/Macros.swift

@@ -9,6 +9,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if hasFeature(Macros) && hasAttribute(attached)
+
 @available(SwiftStdlib 5.9, *)
 @attached(member, names: named(_registrar), named(transactions), named(changes), named(_Storage), named(_storage))
 @attached(memberAttribute)
@@ -20,3 +22,5 @@ public macro Observable() =
 @attached(accessor)
 public macro ObservableProperty() = 
   #externalMacro(module: "ObservationMacros", type: "ObservablePropertyMacro")
+
+#endif

stdlib/public/core/Macros.swift

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if hasFeature(Macros) && hasAttribute(attached)
+
 /// Specifies the module and type name for an externally-defined macro, which
 /// must conform to the appropriate set of `Macro` protocols.
 ///
@@ -90,3 +92,5 @@ public macro dsohandle() -> UnsafeRawPointer = Builtin.DSOHandleMacro
 @attached(conformance)
 public macro OptionSet<RawType>() =
   #externalMacro(module: "SwiftMacros", type: "OptionSetMacro")
+
+#endif