[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

objectivec/GPBStruct.pbobjc.h

diff --git a/objectivec/GPBStruct.pbobjc.h b/objectivec/GPBStruct.pbobjc.h
index 2196639..9bedafc 100644
--- a/objectivec/GPBStruct.pbobjc.h
+++ b/objectivec/GPBStruct.pbobjc.h
@@ -192,4 +192,4 @@
 
 // @@protoc_insertion_point(global_scope)
 
-// clange-format on
+// clang-format on