[Sema] Avoid iterator invalidation when code completing.

It's possible for the code completion consumer to add new decls to the
current scope while lookup happens on it. Avoid this by making a copy
first.

Sadly I wasn't able to get a self-contained test case for this as it
requires code completion + precompiled preamble + the stars aligning to
deserialize at exactly the right time.

llvm-svn: 315772