[ORC] In MaterializationResponsibility, only maintain the Materializing flag on

symbols in debug mode.

The MaterializationResponsibility class hijacks the Materializing flag to track
symbols that have not yet been resolved in order to guard against redundant
resolution. Since this is an API contract check and only enforced in debug mode
there is no reason to maintain the flag state in release mode.

llvm-svn: 334909