[Basic] Always serialize integers in little-endian byte order

This change fixes the ExponentialGrowthAppendingBinaryByteStream
tests on big endian machines.

Force ExponentialGrowthAppendingBinaryByteStreams to use little-
endian byte order. We always used little-endian byte order anyway
and it seems very unlikely we'll need the flexibility to make the
stream big-endian in the future. The benefit of this is that we
can use portable APIs while still allowing the compiler to remove
conditional byte swaps.

Also replace writeRaw with writeInteger and make it explicitly
little-endian to make the API cleaner and more portable.

tools/swift-syntax-test/swift-syntax-test.cpp

@@ -735,8 +735,7 @@ int doSerializeRawTree(const char *MainExecutablePath,
         return EXIT_FAILURE;
       }
 
-      swift::ExponentialGrowthAppendingBinaryByteStream Stream(
-          llvm::support::endianness::little);
+      auto Stream = ExponentialGrowthAppendingBinaryByteStream();
       Stream.reserve(32 * 1024);
       std::map<void *, void *> UserInfo;
       UserInfo[swift::byteTree::UserInfoKeyReusedNodeIds] = &ReusedNodeIds;