[stdlib] Make default implementations of bitwise operators as obsoleted

[moiseev]

Fixes: https://bugs.swift.org/browse/SR-7019

Prior to Swift 4 and the new integers, there was a protocol called
BitwiseOperations. It was deemed not needed with the introduction of
the integer protocols. But, for backward compatibility, it was left in
the standard library as _BitwiseOperations with a conditionally
available typealias BitwiseOperations. That protocol declares only the
"non-muating" operators, the mutating ones (i.e. |= and friends) are
defined as free functions. They are implemented in terms of
_BitwiseOperations protocol requirements, which is reasonable.

In the new integer protocols we established a
convention to provide default implementations for non-mutating functions
in terms of mutating ones (exactly the opposite of a much earlier
decision made for BitwiseOperations). So now, when you define a type
that conforms to the FixedWidthInteger, even though both mutating and
non-mutating bitwise operations are required, the default implementation
of | comes from extension FixedWidthInteger, and the default
implementation of |= comes from the _BitwiseOperations free functions.
And they are mutually recursive. Hence the crash.

This commit breaks the recursion by marking default implementations for
| etc. in _BitwiseOperations obsoleted from Swift 4.1 onward. The
test is also added to make sure the change helps.

[moiseev]

@swift-ci Please smoke test

[moiseev]

@swift-ci Please Test Source Compatibility