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.
