This is a C++ feature that allows the use of `_` to
declare multiple variable of that name in the same scope;
these variables can then not be referred to.
In addition, while P2169 does not extend to parameter
declarations, we stop warning on unused parameters of that name,
for consistency.
The feature is backported to all C++ language modes.
Reviewed By: #clang-language-wg, aaron.ballman
Differential Revision: https://reviews.llvm.org/D153536
We're going to remove the support for modules-ts. But there are a lot of
tests which uses -fmodules-ts. We shouldn't remove them simply. This
patch refactor these tests to use standard c++ modules.
