Create diagnostic group for definition deprecation warning

Create diagnostic group for definition deprecation warning

In https://reviews.llvm.org/D126664, a warning is introduced
warning against the deprecated out of line definition of a
static constexpr member in C++17 and later. Prior to this patch,
the only diagnostic group controlling this diagnostic was -Wdeprecated,
which controls many many diagnostics. This patch creates
a diagnostic group specifically for this warning so it can
be controlled in isolation, while also being included with -Wdeprecated.

Differential Revision: https://reviews.llvm.org/D153881