[RFC] Asynchronous unwind tables attribute

On one hand, we have the `uwtable` attribute in LLVM IR, which tells
whether to emit CFI directives. On the other hand, we have the `clang
-cc1` command-line option `-funwind-tables=1|2 ` and the codegen
option `VALUE_CODEGENOPT(UnwindTables, 2, 0) ///< Unwind tables (1) or
asynchronous unwind tables (2)`.
Thus we lose along the way the information whether we want just some
unwind tables or asynchronous unwind tables.

Asynchronous unwind tables take more space in the runtime image, I'd
estimate something like 80-90% more, as the difference is adding
roughly the same number of CFI directives as for prologues, only a bit
simpler (e.g. `.cfi_offset reg, off` vs. `.cfi_restore reg`). Or even
more, if you consider tail duplication of epilogue blocks.
Asynchronous unwind tables could also restrict code generation to
having only a finite number of frame pointer adjustments (an example
of*not* having a finite number of `SP` adjustments is on AArch64 when
untagging the stack (MTE) in some cases the compiler can modify `SP`
in a loop).
Having the CFI precise up to an instruction generally also means one
cannot bundle together CFI instructions once the prologue is done,
they need to be interspersed with ordinary instructions, which means
extra `DW_CFA_advance_loc` commands, further increasing the unwind
tables size.

That is to say, async unwind tables impose a non-negligible overhead,
yet for the most common use cases (like C++ exceptions), they are not
even needed.

We could, for example, extend the `uwtable` attribute with an optional
value, e.g.
   - `uwtable` (default to 2)
   - `uwtable(1)`, sync unwind tables
   - `uwtable(2)`, async unwind tables
   - `uwtable(3)`, async unwind tables, but tracking only a subset of
registers (e.g. CFA and return address)

Or add a new attribute `async_uwtable`.

Other suggestions? Comments?

Yes, thanks for bringing this up. OpenVMS on x86-64 needs full async
unwind tables as our system allows for exceptions to be handled during
both the prologue and epilogue. We are currently avoiding the poor
quality of prologue/epilogue CFI information at present until we can
move forward to a recent LLVM version (long story) where I think the
support is better.

I like just having a uwtable level (1,2,3,etc).

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