Fix CodeCompletion & TypoCorrection when combining a PCH with Modules

This commit fixes the IdentifierIterator to actually include identifiers
from a PCH or precompiled preamble when there is also a global module
index. This was causing code-completion (outside of C++) and
typo-correction to be missing global identifiers defined in the
PCH/preamble. Typo-correction has been broken since we first started
using the module index, whereas code-completion only started relying on
identifier iterator in r232793.

rdar://problem/25642879

llvm-svn: 268471