Remove IR::Annotations and make IAnnotated to carry annotations inline

[asl]

Lots of cleanups and fixes here and there

[asl]

This is PR on top of #4971 for simplicity.

As mentioned in #4974, IR::Annotations incur a huge overhead, and in general does not make any sense: it's just another level of indirection. So, this just implements old FIXME, plus does some cleanup here and there.

The impact of # of allocations is huge.
Before:

After:

So, we're 10% less of memory allocations by both total size and number.

The runtime impact with GC disabled is huge as well:

Command	Mean [s]	Min [s]	Max [s]	Relative
test/gtestp4c-nogc-main --gtest_filter=P4CParserUnroll.switch_20160512	3.240 ± 0.048	3.195	3.422	1.10 ± 0.03
test/gtestp4c-nogc --gtest_filter=P4CParserUnroll.switch_20160512	2.948 ± 0.061	2.855	3.041	1.00

So, we're 10% faster here (!)

Runtime impact with GC on is much less visible due to huge GC overhead (1.5x):

Command	Mean [s]	Min [s]	Max [s]	Relative
test/gtestp4c-main --gtest_filter=P4CParserUnroll.switch_20160512	4.584 ± 0.099	4.312	4.897	1.02 ± 0.03
test/gtestp4c --gtest_filter=P4CParserUnroll.switch_20160512	4.512 ± 0.072	4.309	4.649	1.00

[asl]

sanitizers are broken after #4989

[ChrisDodd]

Would we get most/all of the memory savings by just using inline here instead of getting rid of IR::Annotations completely? That might be a simpler change.

[asl]

Not all, as IR::Annotations will be still be a Node and it will still be cloned. Though "invisible" now when outer node is cloned by itself. Plus, there was already a FIXME for this change. I think additional wrapper is not required as annotation is a property of the Node itself, so why do we need to have additional level of indirection? For convenience we can certainly do using Annotations = IR::Vector<IR::Annotation> here, yes.

Plus I already fixed couple of bugs when it was Annotations (being a Node) queried for the presence of an Annotation. Certainly the check returned always false. Now only IAnnotated objects could be queried about annotations.

In addition to this: usually we only have 1-2 annotations max (e.g. @name and that's it). So we can store them inline using e.g. absl::InlineVector as underlying storage. This would remove almost all Annotation-related memory allocations. But first we'd need to reduce the size of Allocation itself IMO

[asl]

I rebased the branch after abseil string helpers were merged in. Now it should show the changes more cleanly