Features: Enable the experimental feature CDecl in production

Author: xymus

include/swift/Basic/Features.def

@@ -517,7 +517,7 @@ EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
 EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
 
 /// Allow use of `@cdecl`
-EXPERIMENTAL_FEATURE(CDecl, false)
+EXPERIMENTAL_FEATURE(CDecl, true)
 
 /// Allow use of `@extensible` on public enums
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExtensibleAttribute, false)