Don't emit @_section attributes unless the macro target has SymbolLinkageMarkers. (#1018)

This PR suppresses our use of `@_section` and `@_used` unless the macro
target is built with `SymbolLinkageMarkers` enabled.

Resolves rdar://146819169.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/TestingMacros/Support/TestContentGeneration.swift

@@ -62,7 +62,19 @@ func makeTestContentRecordDecl(named name: TokenSyntax, in typeName: TypeSyntax?
     IntegerLiteralExprSyntax(context, radix: .binary)
   }
 
-  return """
+  var result: DeclSyntax = """
+  @available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
+  private nonisolated \(staticKeyword(for: typeName)) let \(name): Testing.__TestContentRecord = (
+    \(kindExpr), \(kind.commentRepresentation)
+    0,
+    \(accessorName),
+    \(contextExpr),
+    0
+  )
+  """
+
+#if hasFeature(SymbolLinkageMarkers)
+  result = """
   #if hasFeature(SymbolLinkageMarkers)
   #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS) || os(visionOS)
   @_section("__DATA_CONST,__swift5_tests")
@@ -75,13 +87,9 @@ func makeTestContentRecordDecl(named name: TokenSyntax, in typeName: TypeSyntax?
   #endif
   @_used
   #endif
-  @available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
-  private nonisolated \(staticKeyword(for: typeName)) let \(name): Testing.__TestContentRecord = (
-    \(kindExpr), \(kind.commentRepresentation)
-    0,
-    \(accessorName),
-    \(contextExpr),
-    0
-  )
+  \(result)
   """
+#endif
+
+  return result
 }