Sema: resolve necessary information ahead of time

Do the fallible logic in Sema where we have access to error reporting
mechanisms, rather than in Type/Value.

We can't just do the best guess when resolving queries of "is this type
comptime only?" or "what is the ABI alignment of this field?". The
result needs to be accurate. So we need to keep the assertions that the
data is available active, and instead compute the necessary information
before such functions get called.

Unfortunately we are stuck with two versions of such functions because
the various backends need to be able to ask such queries of Types and
Values while assuming the result has already been computed and validated
by Sema.

src/Module.zig

@@ -1125,6 +1125,8 @@ pub const Union = struct {
         abi_align: Value,
 
         /// Returns the field alignment, assuming the union is not packed.
+        /// Keep implementation in sync with `Sema.unionFieldAlignment`.
+        /// Prefer to call that function instead of this one during Sema.
         pub fn normalAlignment(field: Field, target: Target) u32 {
             if (field.abi_align.tag() == .abi_align_default) {
                 return field.ty.abiAlignment(target);