Don't build a call expression referring to a function which we're not allowed

Don't build a call expression referring to a function which we're not allowed
to use. This makes very little difference right now (other than suppressing
follow-on errors in some cases), but will matter more once we support deduced
return types (we don't want expressions with undeduced return types in the
AST).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181107 91177308-0d34-0410-b5e6-96231b3b80d8