)

#### Intro

This is a refactoring of how we perform device code split in
`sycl-post-link`, which is intended to solve several existing issues
with the current implementation:
1. increased peak RAM consumption by `sycl-post-link`
2. bad scaling with more and more split "dimensions" being added
3. increased tests maintenance cost due to non-deterministic order
(between commits) of output files produced by `sycl-post-link`

#### A bit more context about the issues above:

(1) Increase peak RAM consumption is caused by the fact that we
currently preserve **all** splits in-memory, even though we can process
them on-by-one and discard them as soon as we stored them to a disk.
This was implemented as a memory consumption optimization in #5021, but
it got accidentally reverted in #7302 as an attempt to workaround (2).

(2) is pretty much summarized in our source code:

https://github.com/intel/llvm/blob/afebb2543ccecb89f83c84b68fba7616bbab89ac/llvm/tools/sycl-post-link/sycl-post-link.cpp#L806-L811

(3) is caused by a bad implementation decision made in #7302: because
every split is now identified by a hash, every time you add a new split
"dimension"/new feature to an account, it results in different hashes
for existing tests. Just look how many unrelated tests had to be updated
in #7512, #8056 and #8167

#### Now to the PR itself:

It introduces a new infrastructure for categorizing/grouping kernel
functions: instead of using hashes, we now build a string description
for each kernel function and then group kernels with the same
description string together.

String description is built by a new entity: it accepts a set of rules,
where each rule is a simple function which returns a string for passed
`llvm::Function`. Results of all rules are concatenated together and
rules are invoked in a stable order of their registration.

There is a simple API for building those rules. It provides some
predefined rules for the most popular use cases like turning a function
attribute or a metadata into a string descriptor for the function. There
is also a possibility to pass a custom callback there to implement more
complicated logic.

#### How does this PR help with issues above?

(1) and (2) are fixed in conjunction: `sycl-post-link` was refactored to
avoid storing more than one split module at a time and that is possible
because the PR unifies per-scope and optional-kernel-features splitters
into a single generic splitter. The new API for kernels categorization
seems to be flexible enough to provide that infrastructure so merged
splitters still look OK code-wise.

(3) is caused by using string identifiers instead of hashes as well as
by using a data structure which sorts identifiers.

#### Any other benefits from this PR?

About 50 lines of code less to support :)

Extending device code split for more optional features would be even
easier than it is now: instead of adding several changes to various
places around `UsedOptionalFeatures` structure, it will be just adding a
1-3 lines of code. Please also note that `UsedOptionalFeatures` contains
tons of inconsistencies in its implementation, which will all gone with
this PR: in `operator==` we don't use hash and instead compare certain
fields directly (and we do miss some of them); `generateModuleName`
method skips some of optional features and ignores them.

Cross-module `device_global` usages checks should now work at all split
dimensions (except for ESIMD).

#### Any potential downsides?

With current `UsedOptionalFeatures` there is a possibility to embed
various information (used aspects, `large-grf` flag, etc.) directly
during device code split to avoid re-gathering that information later
when we generate properties. With the suggested approach, it would be
harder to do, because it doesn't seem to naturally fit to the proposed
infrastructure: see changes I did around `large-grf` in this PR.

However, we have never actually implemented this and re-querying some
metadata from function doesn't seem like a bottleneck, so it should
really be a very minor and only theoretical downside.