teach FormatDiagnostic to aggregate previously formatted arguments and
pass them down into the ArgToStringFn implementation.  This allows 
redundancy across operands to a diagnostic to be eliminated.

This isn't used yet, so no functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84602 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed