Fix + and - operations on DispatchTime and DispatchWallTime
Fix + and - operations on DispatchTime and DispatchWallTime

... for the cases where seconds parameter is NaN or infinite.
rdar://problem/29764171

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch