Add noexcept to missing places in expected.h

These move and assignment operations are conditionally noexcept, so
add that to their definitions.  This is a performance issue as well as
correctness, since std containers will copy instead of move during
resize if these operations are not noexcept.

Test: build
Change-Id: I148f7eb3489e7f1dd68cc0fb0e555b56470e42da