Disable a couple of warnings for MSVC in folly::Function

Disable a couple of warnings for MSVC in folly::Function

Summary:
MSVC generates warnings if you define multiple assignment operators or multiple copy constructors on the same type. This is deliberate in `folly::Function`, so just disable the warnings for the struct instead.
Note that the warnings have to be disabled for the entire struct for them to actually be disabled. Disabling them just around the declarations of the constructors and operators in question does not actually disable the warnings.

Reviewed By: yfeldblum

Differential Revision: D3347746

fbshipit-source-id: abb53a1e62bcfb7ce02759a7ce8637d824a82081