Suppress MSVC warning 4250 about "inherits via dominance"
Suppress MSVC warning 4250 about "inherits via dominance"

Summary: `ThreadManagerExecutorAdapter` and `SimpleThreadManager` inherit from both `folly::Executor` and `folly::DefaultKeepAliveExecutor` (`Executor`'s child). The methods `keepAliveAcquire` and `keepAliveRelease` are defined in both of those  classes. By C++ rules, this means that the version in the child is the one that is used in these further derived classes, but this triggers a warning in MSVC that spams the logs. This code pattern cannot be easily changed by, for example, instantiating these functions in the `ThreadManager` classes and invoking the version from `DefaultKeepAliveExecutor` because these are private methods and can't be explicitly invoked from the child class. So, instead, we will suppress the warnings.

Reviewed By: skotchvail

Differential Revision: D31758863

fbshipit-source-id: ae0eaa27f1605dfdc0b28ba74d4e85fc903b686e

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