lint: add test for treating _Bool as non-scalar type

This strict mode is not yet implemented.  The plan is to use it for
usr.bin/make, to get rid of the many possible variants of defining the
Boolean type in make.h.  These variants did find some bugs, but not
reliably so.  Using static analysis seems more promising for this.

In an early stage of developing this test, lint1 crashed in the enum
definition in line 213, where the node for the '?:' had been NULL.  This
can happen in other situations as well, such as with syntax errors, but
these should be rare, as lint is usually only run if the compiler has
accepted the source code.  Still, there should not be any assertion
failures while running lint1.

(rillig)