* Don't include <cassert>. (#2148)
This commit replaces use of the assert() macro in format-inl.h with
FMT_ASSERT(). This allows us to drop the cassert include.
* FMT_GCC_VERSION is not defined when we include test-assert.h, use __GCC__ instead.
* Don't explicitly suppress GCC's -Wterminate in tests' FMT_ASSERT.
Throwing from a separate function is enough to silence the warning, no need to
explicitly suppress it.
* Remove messages from assertions added in 2f699d2.
* Correct formatting around throw_assertion_failure().
* Avoid weak vtables by providing a private virtual member function
* Add warning Wweak-vtables to clang when FMT_PEDANTIC is on
* Add defaulted copy and move operations to format_error and system_error
Compiler generated copy operations are deprecated and move operations
are not generated altogether.
* Add warning Wdeprecated to clang when FMT_PEDANTIC is on
