Add unit tagging to ExprRegionId (#6272)

This doesn't show up in the raw SemIR dumps (I don't think that's due to
a lack of coverage, but due to the fact that ExprRegionId isn't used as
an operand of any instructions).

Author: dwblaikie

toolchain/sem_ir/file.cpp

@@ -62,6 +62,7 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       constants_(this),
       // 1 reserved id for `StructTypeFields::Empty`.
       struct_type_fields_(allocator_, IdTag(check_ir_id.index, 1)),
+      expr_regions_(check_ir_id),
       clang_source_locs_(check_ir_id) {
   // `type` and the error type are both complete & concrete types.
   types_.SetComplete(