[Modules] Early-exit if ReadOptionsBlock fails to avoid crashing

If we didn't tell ReadOptionsBlock to allow failures then we can't
assume that the stream is not in the middle of a block if it returns
out-of-date. This was causing a crash when we tried to continue reading.

Also, it's just generally a good idea to early-exit if we're doing
implicit module builds, since we will want to immediately rebuild this
module anyway and there's no reason to waste time continuing after
failure.

rdar://problem/24114938

llvm-svn: 260563