tint: optimize compile time for const_eval_*_test files

The reason for slow compile times is because the very large variants of
builder::Value<T>s combined with the many std::visits over these
variants result in many combinatorial instantiations of the visit
callbacks.

To address this, I added a polymorphic base class ValueBase to Value<T>,
and replaced most of the std::visit-based compile time code with runtime
virtual calls. For the two heaviest users of std::visit over the large
variants, compiles times dropped more than half (clang-10, debug):

const_eval_binary_op_test.cc: 19.079s to 7.736s
const_eval_unary_op_test.cc: 10.021s to 4.789s

Bug: tint:1711
Change-Id: Iba05e6ae1004ef0814250e2a8ea50aa2b26b85f2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/105782
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>