Add comment explaining IExtensionMessage instability

 * Add Syntax enum and make file syntax checks check the enum
 * Made IsPacked a normal get method without ternary op
 * Moved IFieldAccessor.HasValue to IFieldPresenceAccessor
 * Change naming of GetIsExtensionsInitialized
 * Fixed stray text in summary text

Add ExtensionSet tests and add checks that non-message extension fields don't fail initialization checks

Adjust APIs for extensions to properly return default values for extensions
Fix issues with IsInitialized and proto2 field reflection

Fix generated group code missing end group tag case
Fix generated field classes searching for presence index for extensions

Removed ByteCountInt64.  This is a transitional typedef that should n…

* Test singular fields are encoded in canonical way

* Defautl values in proto3 should not be encoded.
* Values should be converted to the canonical representation (e.g.,
long int64 value may be truncated for int32 field)

* Update failure list

* Update failure list

clang-cl requires the order to be the opposite of what it currently is.
It should not affect other compilers since this is the only compiler
that has both macros defined.