[Sema] Avoid iterator invalidation when code completing.

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315772 91177308-0d34-0410-b5e6-96231b3b80d8