(motivated by test failures after D157058)
With D157058 the base template for `std::char_traits` was removed from
libc++. Quoting the release notes:
```
The base template for ``std::char_traits`` has been removed. If you are using
``std::char_traits`` with types other than ``char``, ``wchar_t``, ``char8_t``,
``char16_t``, ``char32_t`` or a custom character type for which you
specialized ``std::char_traits``, your code will no longer work.
```
This patch simply removes all such instantiations to make sure the
tests that run against the latest libc++ version pass.
One could try testing the existence of this base template from within
the test source files but this doesn't seem like something we want
support.
Differential Revision: https://reviews.llvm.org/D157636
487ab39a50