[TableGen] Use guarded assert in constexpr functions (#194728)
The constexpr functions in question take a scoped enum as an argument
and a switch statement returns a value for each value of the enum. These
are all legal statements in a constexpr function in C++14.
Under constexpr rules, the evaluation of a constexpr function cannot
lead to an evaluation of any prohibited forms of expressions. An
evaluation of the functions being discussed with a valid argument will
terminate at the switch, and an code that follows will not be evaluated.
Using "llvm_unreachable" after the switch should be ok as long as the
expansion of the llvm_unreachable macro does not contain any statements
not allowed to appear in a constexpr function. At the same time, GCC
before v9 did not tolerate any unguarded calls to non-constexpr
functions after the switch.
To avoid using "llvm_unreachable", which can have multiple expansions,
use an assert with an explicit condition that the underlying value of
the argument lies between the minimum and maximum values of the enum.
