Fix a use-after-free found in libclang when doing code completion. The
loop processing the candidates can cause new declerations to be added to
the context, invalidating lookup_result. To avoid that, make a copy of
the list of declarations to iterate over.

I don't have a way to check in a test case for this as it involves
a giant pile of source code and a generated PCH file used to accelerate
code completion, all of this running under ASan.