[Refactor NFC] Rename the (non-CCE, fold-failure) Diag during constant expression...

[Refactor NFC] Rename the (non-CCE, fold-failure) Diag during constant expression evaluation as FFDiag.

Currently, we have CCEDiags (C++11 core constant expression diags) and Fold failure diagnostics [I don't claim to yet fully understand exactly why we need the difference].  This patch explicitly replaces Info.Diag (whose use always represents a fold failure diag within the file) with Info.FFDiag.  This makes it more easily greppable in the file, and just like the name Info.CCEDiag, it gives the reader slight further insight into the nature of the diagnostic (as opposed to Info.Diag).

This patch is a preliminary refactoring step in an effort to allow support for compatibility-warnings and extensions (such as constexpr lambda) during constant expression evaluation.

All regressions pass.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274454 91177308-0d34-0410-b5e6-96231b3b80d8