[Sema] Deduplicate some availability checking logic

Authored by erik.pilkington on Mar 20 2019, 12:26 PM.


[Sema] Deduplicate some availability checking logic

Before this commit, we emit unavailable errors for calls to functions during
overload resolution, and for references to all other declarations in
DiagnoseUseOfDecl. The early checks during overload resolution aren't as good as
the DiagnoseAvailabilityOfDecl based checks, as they error on the code from
PR40991. This commit fixes this by removing the early checking.


Differential revision: https://reviews.llvm.org/D59394

llvm-svn: 356599