There was a cyclic dependency: For RTEMS_STATIC_ANALYSIS we needed
basedefs.h in assert.h. For RTEMS_UNREACHABLE() we needed _Assert() from
assert.h in basedefs.h.
Fix this by introducing _Debug_Unreachable() in basedefs.h.
Add RTEMS_FUNCTION_NAME to basedefs.h and use it in basedefs.h and
assert.h.
Close#4900.