[ObjC] Move GenerateHeader over to Printer::Emit.

Update ImportWriter to have a second Emit api for the runtime files to
streamline things a bit better.

PiperOrigin-RevId: 492210147
diff --git a/objectivec/GPBDuration.pbobjc.h b/objectivec/GPBDuration.pbobjc.h
index 223d262..05c63d8 100644
--- a/objectivec/GPBDuration.pbobjc.h
+++ b/objectivec/GPBDuration.pbobjc.h
@@ -133,4 +133,4 @@
 
 // @@protoc_insertion_point(global_scope)
 
-// clange-format on
+// clang-format on