Those creep up from time to time. We need to use `int main(int, char**)`
because in freestanding mode, `main` doesn't get special treatment and
special mangling, so we setup a symbol alias from the mangled version of
`main(int, char**)` to `extern "C" main`. That only works if all the tests
are consistent about how they define their main function.
All supported compilers have supported deduction guides in C++17 for a
while, so this isn't necessary anymore.
Differential Revision: https://reviews.llvm.org/D108213
Implement the resolution of LWG2993. Replace a deleted constructor
with a constructor that SFINAEs away in appropriate circumstances.
Also, now that the constructor is templated, we must have an
explicit deduction guide to make CTAD work.
Some tests have been merged in from Agustín Bergé's D40259.
Differential Revision: https://reviews.llvm.org/D92725
Louis Dionne