libexpr/primops: Suppress deprecation warning in prim_importNative

as of cl/5456 prim_importNative default constructs a `Value` in order to
pass as an out parameter to a ValueInitializer function. Default
construction is deprecated, but there isn't a meaningful value to
initialize it to instead. After cl/5357 this will give a deprecation
warning (which in an asanBuild will be an error), so this commit
suppresses that warning for that `Value` construction.

Change-Id: Ie567a6e81b5753672001320e9fc816c56a6a6964

lix/libexpr/primops.cc

@@ -300,8 +300,16 @@ Value prim_importNative(EvalState & state, Value ** args)
             state.ctx.errors.make<EvalError>("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected", sym, path).debugThrow();
     }
 
+    // Default construction of `Value` is deprecated, and uses of `Value&` out parameters
+    // have mostly been removed in favor of returning a `Value` instead. However in this
+    // particular case, this signature (ValueInitializer defined above) is externally visible,
+    // changing it would be an API breaking change, so for this one instance we just suppress
+    // the warning instead.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
     Value v;
     (func)(state, v);
+#pragma clang diagnostic pop
 
     /* We don't dlclose because v may be a primop referencing a function in the shared object file */
     return v;