Add FlatBufferBuilder move semantics tests to the main test suite (#4902)

* Add FlatBufferBuilder move semantics tests to main

Do not eagerly delete/reset allocators in release and release_raw functions
Update android, vs2010 build files
New tests for various types of FlatBufferBuilders and move semantics

* Improve test failure output with function names

include/flatbuffers/flatbuffers.h

@@ -581,10 +581,10 @@ class vector_downward {
       buf_(other.buf_),
       cur_(other.cur_),
       scratch_(other.scratch_) {
-    other.allocator_ = nullptr;
-    other.own_allocator_ = false;
+    // No change in other.allocator_
     // No change in other.initial_size_
     // No change in other.buffer_minalign_
+    other.own_allocator_ = false;
     other.reserved_ = 0;
     other.buf_ = nullptr;
     other.cur_ = nullptr;
@@ -639,18 +639,22 @@ class vector_downward {
     allocated_bytes = reserved_;
     offset = static_cast<size_t>(cur_ - buf_);
 
+    // release_raw only relinquishes the buffer ownership.
+    // Does not deallocate or reset the allocator. Destructor will do that.
     buf_ = nullptr;
-    clear_allocator();
     clear();
     return buf;
   }
 
   // Relinquish the pointer to the caller.
   DetachedBuffer release() {
+    // allocator ownership (if any) is transferred to DetachedBuffer.
     DetachedBuffer fb(allocator_, own_allocator_, buf_, reserved_, cur_,
                       size());
-    allocator_ = nullptr;
-    own_allocator_ = false;
+    if (own_allocator_) {
+      allocator_ = nullptr;
+      own_allocator_ = false;
+    }
     buf_ = nullptr;
     clear();
     return fb;

include/flatbuffers/grpc.h

@@ -89,13 +89,15 @@ class SliceAllocator : public Allocator {
   SliceAllocator &operator=(const SliceAllocator &other) = delete;
 
   SliceAllocator(SliceAllocator &&other)
-    : slice_(other.slice_) {
-    other.slice_ = grpc_empty_slice();
+    : slice_(grpc_empty_slice()) {
+    // default-construct and swap idiom
+    swap(other);
   }
 
   SliceAllocator &operator=(SliceAllocator &&other) {
-    slice_ = other.slice_;
-    other.slice_ = grpc_empty_slice();
+    // move-construct and swap idiom
+    SliceAllocator temp(std::move(other));
+    swap(temp);
     return *this;
   }
 
@@ -190,6 +192,16 @@ class MessageBuilder : private detail::SliceAllocatorMember,
     buf_.swap_allocator(other.buf_);
   }
 
+  // Releases the ownership of the buffer pointer.
+  // Returns the size, offset, and the original grpc_slice that
+  // allocated the buffer. Also see grpc_slice_unref().
+  uint8_t *ReleaseRaw(size_t &size, size_t &offset, grpc_slice &slice) {
+    uint8_t *buf = FlatBufferBuilder::ReleaseRaw(size, offset);
+    slice = slice_allocator_.slice_;
+    slice_allocator_.slice_ = grpc_empty_slice();
+    return buf;
+  }
+
   ~MessageBuilder() {}
 
   // GetMessage extracts the subslice of the buffer corresponding to the