Check for use of InstIds from the wrong SemIR::File (#5997)

Use the `CheckIRId` as a unique identifier for the scope of an `InstId`
- if an `InstId` is created within the scope of one `CheckIRId` it must
not be used in the scope of a different `CheckIRId`.

This is achieved without extra storage, but with false negatives for
large inputs.

When an `InstId` is created, the original index of the `Inst` is XORed
with a tag derived from the `CheckIRId` to produce the final `InstId`.
When the `InstId` is used, the expected tag is XORed with the `InstId`
to get back to the original index - if the tags don't match, the
resulting index will be corrupted, likely too large - resulting in an
out of bounds index CHECK-failure.

(the tag value is derived as such:
* take the CheckIRId
* left shift one bit (padding zero)
* left shift another bit (padding 1 - used to signify that the resulting
`InstId` has a tag combined into it)
* reverse the bits

In this way, the tag is unlikely to overlap with the index for small
test cases - making it possible to separate out the `CheckIRId` from the
index in these cases to provide more meaningful debugging/CHECK
messages, and more informative `SemIR` textual dumping that can now
include the `CheckIRId` along with the `Inst`'s index in the name of an
`inst`)

The test churn here is improved printing as tagged `InstId`s can now,
with best effort (more likely for small test cases where the `CheckIRId`
and the `Inst` index aren't at risk of overlapping from the high and low
bits), render the `CheckIRId` as part of the inst's name. Going from
`instNN` to `irMM.instNN`.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/language_server/context.cpp

@@ -168,7 +168,7 @@ auto Context::File::SetText(Context& context, std::optional<int64_t> version,
   Check::CheckParseTreesOptions check_options;
   check_options.vlog_stream = context.vlog_stream();
   auto getters =
-      Parse::GetTreeAndSubtreesStore::MakeWithExplicitSize(1, getter);
+      Parse::GetTreeAndSubtreesStore::MakeWithExplicitSize(IdTag(), 1, getter);
 
   auto clang_invocation =
       BuildClangInvocation(consumer, fs, {context.installation().clang_path()});