static_assert declarations have to be visited while indexing so that we can
gather the references to declarations that are present in their assert
expression.

Differential Revision: https://reviews.llvm.org/D33913


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305504 91177308-0d34-0410-b5e6-96231b3b80d8