libstdc++: Tell GCC what basic_string::_M_is_local() means [PR109299]

libstdc++: Tell GCC what basic_string::_M_is_local() means [PR109299]

This avoids a bogus warning about overflowing a buffer, because GCC
can't tell that we don't copy into the buffer unless it fits. By adding
a __builtin_unreachable() hint we inform the compiler about the
invariant that the buffer is only used when it's big enough.

This can also improve codegen, by eliminating dead code that GCC
couldn't tell was unreachable.

libstdc++-v3/ChangeLog:

	PR libstdc++/109299
	* include/bits/basic_string.h (basic_string::_M_is_local()): Add
	hint for compiler that local strings fit in the local buffer.