Re-factor: Change AstGen.ResultLoc to be a struct

This re-factor is intended to make it easier to track what kind of
operator/expression consumes a result location, without overloading the
ResultLoc union for this purpose.

This is used in the following commit to keep track of initializer
expressions of `const` variables to avoid popping error traces
pre-maturely. Hopefully this will also be useful for implementing
RLS temporaries in the future.

test/behavior/bugs/12891.zig

@@ -7,6 +7,7 @@ test "issue12891" {
     try std.testing.expect(i < f);
 }
 test "nan" {
+    if (builtin.zig_backend == .stage1) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
 
     const f = comptime std.math.nan(f64);