Boost 1.70.0 broke the apply_visitor functions for lvalue reference
variants.
This imports the patch that fixes this issue from upstream.

Tested this by compiling a library
(https://github.com/facebookincubator/fizz) that works with 1.69 but
breaks with 1.70. And then, importing this patch and trying the
compilation again.

Compile tested: nbg6817

Maintainer: @ClaymorePT

Signed-off-by: Amol Bhave <ambhave@fb.com>