swift234

Eliminate UnsafeMutableBufferPointer from _withUnsafeMutableBufferPointerIfSupported

The type checker hits a recursion when checking the conformance to
CollectionType in UnsafeMutableBufferPointer, which requires
_withUnsafeMutableBufferPointerIfSupported, which mentions
UnsafeMutableBufferPointer.  The easiest fix for now is to break the
recursion in the library.

Reverting this change is tracked by: <rdar://problem/21933004> Restore
the signature of _withUnsafeMutableBufferPointerIfSupported() that
mentions UnsafeMutableBufferPointer

Swift SVN r30838