Add more warnings to for the ObjC runtime build

Working on https://github.com/google/protobuf/issues/1599, specifically:
- Turn on more warnings that the Xcode UI calls out with individual controls.
- Manually add:
  -Wundef
  -Wswitch-enum
- Manually add and then diable in the unittests because of XCTest's headers:
  -Wreserved-id-macro
  -Wdocumentation-unknown-command
- Manually add -Wdirect-ivar-access, but disable it for the unittests and in
  the library code (via #pragmas to suppress it). This is done so proto users
  can enable the warning.

objectivec/GPBDescriptor_PackagePrivate.h

diff --git a/objectivec/GPBDescriptor_PackagePrivate.h b/objectivec/GPBDescriptor_PackagePrivate.h
index e3d0a80..c20ff6b 100644
--- a/objectivec/GPBDescriptor_PackagePrivate.h
+++ b/objectivec/GPBDescriptor_PackagePrivate.h
@@ -245,6 +245,12 @@
 
 CF_EXTERN_C_BEGIN
 
+// Direct access is use for speed, to avoid even internally declaring things
+// read/write, etc. The warning is enabled in the project to ensure code calling
+// protos can turn on -Wdirect-ivar-access without issues.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdirect-ivar-access"
+
 GPB_INLINE BOOL GPBFieldIsMapOrArray(GPBFieldDescriptor *field) {
   return (field->description_->flags &
           (GPBFieldRepeated | GPBFieldMapKeyMask)) != 0;
@@ -262,6 +268,8 @@
   return field->description_->number;
 }
 
+#pragma clang diagnostic pop
+
 uint32_t GPBFieldTag(GPBFieldDescriptor *self);
 
 // For repeated fields, alternateWireType is the wireType with the opposite
@@ -291,23 +299,23 @@
 }
 
 // Helper for compile time assets.
-#ifndef _GPBCompileAssert
+#ifndef GPBInternalCompileAssert
   #if __has_feature(c_static_assert) || __has_extension(c_static_assert)
-    #define _GPBCompileAssert(test, msg) _Static_assert((test), #msg)
+    #define GPBInternalCompileAssert(test, msg) _Static_assert((test), #msg)
   #else
     // Pre-Xcode 7 support.
-    #define _GPBCompileAssertSymbolInner(line, msg) _GPBCompileAssert ## line ## __ ## msg
-    #define _GPBCompileAssertSymbol(line, msg) _GPBCompileAssertSymbolInner(line, msg)
-    #define _GPBCompileAssert(test, msg) \
-        typedef char _GPBCompileAssertSymbol(__LINE__, msg) [ ((test) ? 1 : -1) ]
+    #define GPBInternalCompileAssertSymbolInner(line, msg) GPBInternalCompileAssert ## line ## __ ## msg
+    #define GPBInternalCompileAssertSymbol(line, msg) GPBInternalCompileAssertSymbolInner(line, msg)
+    #define GPBInternalCompileAssert(test, msg) \
+        typedef char GPBInternalCompileAssertSymbol(__LINE__, msg) [ ((test) ? 1 : -1) ]
   #endif  // __has_feature(c_static_assert) || __has_extension(c_static_assert)
-#endif // _GPBCompileAssert
+#endif // GPBInternalCompileAssert
 
 // Sanity check that there isn't padding between the field description
 // structures with and without a default.
-_GPBCompileAssert(sizeof(GPBMessageFieldDescriptionWithDefault) ==
-                  (sizeof(GPBGenericValue) +
-                   sizeof(GPBMessageFieldDescription)),
-                  DescriptionsWithDefault_different_size_than_expected);
+GPBInternalCompileAssert(sizeof(GPBMessageFieldDescriptionWithDefault) ==
+                         (sizeof(GPBGenericValue) +
+                          sizeof(GPBMessageFieldDescription)),
+                         DescriptionsWithDefault_different_size_than_expected);
 
 CF_EXTERN_C_END