[REVISED PATCH 4/9]: C++ P0482R5 char8_t: Updates to existing core language tests

[Mike Stump]

I'd like to punt to Jason for this patch.

> On Dec 23, 2018, at 6:27 PM, Tom Honermann <tom@honermann.net> wrote:
> 
> Attached is a revised patch that addresses changes in P0482R6 and adoption of P0482R6 for C++20 in San Diego.  Changes from the prior patch include:
> - Updated a test to validate the value of the __cpp_char8_t feature test
>  macro when targeting C++2a.