This patch uses the previously build infrastructure to parse multiple
FDE entries into a single unwind plan. There is one catch though: we
parse only one FDE entry per unwind range. This is not fully correct
because lldb coalesces adjecant address ranges, which means that
something that originally looked like two separate address ranges (and
two FDE entries) may get merged into one because if the linker decides
to put the two ranges next to each other. In this case, we will ignore
the second FDE entry.

It would be more correct to try to parse another entry when the one we
found turns out to be short, but I'm not doing this (yet), because:
- this is how we've done things so far (although, monolithic functions
are unlikely to have more than one FDE entry)
- in cases where we don't have debug info or (full) symbol tables, we
can end up with "symbols" which appear to span many megabytes
(potentially, the whole module). If we tried to fill short FDE entries,
we could end up parsing the entire eh_frame section in a single go. In a
way, this would be more correct, but it would also probably be very
slow.

I haven't quite decided what to do about this case yet, though it's not
particularly likely to happen in the "production" cases as typically the
functions are split into two parts (hot/cold) instead of one part per
basic block.

This is mainly useful for discontinuous functions because individual
parts of the function will have separate FDE entries, which can begin
many megabytes from the start of the function. However, I'm separating
it out, because it turns out we already have a test case for the
situation where the FDE does not begin exactly at the function boundary.

The test works mostly by accident because the FDE starts only one byte
after the beginning of the function so it doesn't really matter whether
one looks up the unwind row using the function or fde offset. In this
patch, I beef up the test to catch this problem more reliably.

To make this work I've also needed to change a couple of places which
that an unwind plan always has a row at offset zero.

.. which prepares us for handling of discontinuous functions. The main
change there is that we can have multiple FDEs contributing towards an
unwind plan of a single function. This patch separates the logic for
parsing of a single FDE from the construction of an UnwindPlan (so that
another patch can change the latter to combine several unwind plans).

Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>

These are needed for functions whose entry point is not their lowest
address.

`GetLSDAAddress` and `GetPersonalityRoutinePtrAddress` are unused and
they create a bit of a problem for discontinuous functions, because the
unwind plan for these consists of multiple eh_frame descriptors and (at
least in theory) each of them could have a different value for these
entities.

We could say we only support functions for which these are always the
same, or create some sort of a Address2LSDA lookup map, but I think it's
better to leave this question to someone who actually needs this.

To be able to describe discontinuous functions, this patch changes the
UnwindPlan to accept more than one address range.

I've also squeezed in a couple improvements/modernizations, for example
using the lower_bound function instead of a linear scan.

In most places, the rows are copied anyway (because they are generated
by cumulating modifications) immediately after adding them to the unwind
plans. In others, they can be moved into the unwind plan. This lets us
remove some backflip copies and make `const UnwindPlan` actually mean
something.

I've split this patch into two (and temporarily left both APIs) as this
patch was getting a bit big. This patch covers all the interesting
cases. Part two all about converting "architecture default" unwind plans
from ABI and InstructionEmulation plugins.

lldb has two RegisterLocation classes that do slightly different things.

UnwindPlan::Row::RegisterLocation (new: AbstractRegisterLocation) has a
description of how to find a register's value or location, not specific
to a particular stopping point. It may say that at a given offset into a
function, the caller's register has been spilled to stack memory at CFA
minus an offset. Or it may say that the caller's register is at a DWARF
exprssion.

UnwindLLDB::RegisterLocation (new: ConcreteRegisterLocation) is a
specific address where the register is currently stored, or the register
it has been copied into, or its value at this point in the current
function execution.

When lldb stops in a function, it interprets the
AbstractRegisterLocation's instructions using the register context and
stack memory, to create the ConcreteRegisterLocation at this point in
time for this stack frame.

I'm not thrilled with AbstractRegisterLocation and
ConcreteRegisterLocation, but it's better than the same name and it will
be easier to update them if someone suggests a better pair.

This PR is in reference to porting LLDB on AIX.

Link to discussions on llvm discourse and github:
1.  https://discourse.llvm.org/t/port-lldb-to-ibm-aix/80640
2.  #101657 

The complete changes for porting are present in this draft PR:
https://github.com/llvm/llvm-project/pull/102601

The changes on this PR are intended to avoid namespace collision for
certain typedefs between lldb and other platforms:
1. tid_t --> lldb::tid_t
2. offset_t --> lldb::offset_t

The macro already uses LLVM_PRETTY_FUNCTION as the timer category, so
there's no point in duplicating it in the timer message.