Use references to Objective C classes instead of looking classes up by name.

This should reduce binary size slightly, small performance improvement, and improve linkage by forcing references to all used classes.

Note that this maintains backwards compatibility for sources generated by older protoc for the time being. If you want the benefits
you will need to recompile your protos with the newer protoc.

objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj

diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
index 9f1d825..1b60be2 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
@@ -36,6 +36,7 @@
 		8BBEA4BB147C729200C4ADB7 /* libProtocolBuffers.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 7461B52E0F94FAF800A0C422 /* libProtocolBuffers.a */; };
 		8BD3981F14BE59D70081D629 /* GPBUnittestProtos.m in Sources */ = {isa = PBXBuildFile; fileRef = 8BD3981E14BE59D70081D629 /* GPBUnittestProtos.m */; };
 		8BF8193514A0DDA600A2C982 /* Foundation.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 1D30AB110D05D00D00671497 /* Foundation.framework */; };
+		8BFF9D1C23AD593C00E63E32 /* GPBMessageTests+ClassNames.m in Sources */ = {isa = PBXBuildFile; fileRef = 8BFF9D1B23AD593B00E63E32 /* GPBMessageTests+ClassNames.m */; };
 		F401DC351A8E5C6F00FCC765 /* GPBArrayTests.m in Sources */ = {isa = PBXBuildFile; fileRef = F401DC341A8E5C6F00FCC765 /* GPBArrayTests.m */; };
 		F40EE4F0206BF91E0071091A /* GPBCompileTest01.m in Sources */ = {isa = PBXBuildFile; fileRef = F40EE4CD206BF9170071091A /* GPBCompileTest01.m */; };
 		F40EE4F1206BF91E0071091A /* GPBCompileTest02.m in Sources */ = {isa = PBXBuildFile; fileRef = F40EE4C6206BF9170071091A /* GPBCompileTest02.m */; };
@@ -183,6 +184,7 @@
 		8BD3981D14BE54220081D629 /* unittest_enormous_descriptor.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = unittest_enormous_descriptor.proto; path = ../../src/google/protobuf/unittest_enormous_descriptor.proto; sourceTree = "<group>"; };
 		8BD3981E14BE59D70081D629 /* GPBUnittestProtos.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBUnittestProtos.m; sourceTree = "<group>"; };
 		8BEB5AE01498033E0078BF9D /* GPBRuntimeTypes.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GPBRuntimeTypes.h; sourceTree = "<group>"; };
+		8BFF9D1B23AD593B00E63E32 /* GPBMessageTests+ClassNames.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = "GPBMessageTests+ClassNames.m"; sourceTree = "<group>"; };
 		F401DC341A8E5C6F00FCC765 /* GPBArrayTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBArrayTests.m; sourceTree = "<group>"; };
 		F40EE4C2206BF9160071091A /* GPBCompileTest08.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBCompileTest08.m; sourceTree = "<group>"; };
 		F40EE4C3206BF9160071091A /* GPBCompileTest04.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBCompileTest04.m; sourceTree = "<group>"; };
@@ -491,6 +493,7 @@
 				7461B6A30F94FDF800A0C422 /* GPBMessageTests.m */,
 				F4487C841AAF6AC500531423 /* GPBMessageTests+Merge.m */,
 				F4487C761AADF84900531423 /* GPBMessageTests+Runtime.m */,
+				8BFF9D1B23AD593B00E63E32 /* GPBMessageTests+ClassNames.m */,
 				F4487C801AAF62FC00531423 /* GPBMessageTests+Serialization.m */,
 				F4B51B1B1BBC5C7100744318 /* GPBObjectiveCPlusPlusTest.mm */,
 				F41C175C1833D3310064ED4D /* GPBPerfTests.m */,
@@ -663,6 +666,7 @@
 			developmentRegion = English;
 			hasScannedForEncodings = 1;
 			knownRegions = (
+				English,
 				en,
 			);
 			mainGroup = 29B97314FDCFA39411CA2CEA /* CustomTemplate */;
@@ -777,6 +781,7 @@
 				8B4248B41A8BD96E00BC1EC6 /* GPBSwiftTests.swift in Sources */,
 				F40EE512206C068D0071091A /* GPBCompileTest25.m in Sources */,
 				F4584D831ECCB53600803AB6 /* GPBExtensionRegistryTest.m in Sources */,
+				8BFF9D1C23AD593C00E63E32 /* GPBMessageTests+ClassNames.m in Sources */,
 				5102DABC1891A073002037B6 /* GPBConcurrencyTests.m in Sources */,
 				F4487C771AADF84900531423 /* GPBMessageTests+Runtime.m in Sources */,
 				F40EE4F1206BF91E0071091A /* GPBCompileTest02.m in Sources */,
@@ -851,6 +856,7 @@
 			buildSettings = {
 				CLANG_ENABLE_MODULES = YES;
 				CLANG_ENABLE_OBJC_WEAK = YES;
+				ENABLE_BITCODE = YES;
 				FRAMEWORK_SEARCH_PATHS = (
 					"\"$(DEVELOPER_LIBRARY_DIR)/Frameworks\"",
 					"$(inherited)",
@@ -883,6 +889,7 @@
 			buildSettings = {
 				CLANG_ENABLE_MODULES = YES;
 				CLANG_ENABLE_OBJC_WEAK = YES;
+				ENABLE_BITCODE = YES;
 				FRAMEWORK_SEARCH_PATHS = (
 					"\"$(DEVELOPER_LIBRARY_DIR)/Frameworks\"",
 					"$(inherited)",