Implement a framework for the delay of arbitrary diagnostics within

Implement a framework for the delay of arbitrary diagnostics within
templates.  So delay access-control diagnostics when (for example) the target
of a friend declaration is a specific specialization of a template.

I was surprised to find that this was required for an access-controlled selfhost.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99383 91177308-0d34-0410-b5e6-96231b3b80d8