Partial fix for PR3310, concerning type-checking for tentative

Partial fix for PR3310, concerning type-checking for tentative
definitions. We were rejecting tentative definitions of incomplete
(which is bad), and now we don't.

This fix is partial because we don't do the end-of-translation-unit
initialization for tentative definitions that don't ever have any
initializers specified.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66584 91177308-0d34-0410-b5e6-96231b3b80d8