[BitwiseCopyable] Only deprecate underscored.

Rather than immediately outright banning the old protocol, just make it
be a deprecated typealias for the real one.

Author: nate-chandler

stdlib/public/core/Misc.swift

@@ -180,5 +180,5 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
 @_marker public protocol BitwiseCopyable { }
 #endif
 
-@available(*, unavailable)
-@_marker public protocol _BitwiseCopyable {}
+@available(*, deprecated, message: "Use BitwiseCopyable")
+public typealias _BitwiseCopyable = BitwiseCopyable

test/Sema/bitwse_copyable_underscore.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift                       \
+// RUN:     -disable-availability-checking                   \
+// RUN:     -debug-diagnostic-names
+
+struct S : _BitwiseCopyable {} // expected-warning {{'_BitwiseCopyable' is deprecated: Use BitwiseCopyable}}
+
+func f<T : _BitwiseCopyable>(_ t: T) {} // expected-warning {{'_BitwiseCopyable' is deprecated: Use BitwiseCopyable}}