[codecomplete] Attempt to workaround ArchetypeBuilder crash in non-asserts build
[codecomplete] Attempt to workaround ArchetypeBuilder crash in non-asserts build

Add a defensive return on null if we're building without assertions.
There appears to be a way to trigger this in code-completion, so until
we find a reproducer/figure out what's going wrong, try not to crash.
Only impacts no-asserts builds.

rdar://problem/27615954

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