[libc++][test] Change forbidden `extents<char>` to `extents<signed char>` (#73535)

Found while running libc++'s test suite with MSVC's STL.

[mdspan.extents.overview]/1.1 mandates that IndexType is a signed or
unsigned integer type, which excludes char.

MSVC's STL enforces the Mandates here, so this PR changes the relevant
occurrences of `char to `signed char`. To make this work, we also need
to add an `operator signed char()` to the test helper type `IntType` so
it remains unambiguously convertible, and then we can remove `operator
char()`.

libc++ should also enforce the Mandates, but this PR doesn't attempt to make
such a change.