We typically shouldn't get this, but when we do (e.g. in #139439) we
should error out gracefully instead of crashing.

Note that we are stricter than ld64 here; ld64 appears to be able to
handle section offsets that point outside literal sections if the end
result is a valid pointer to another section in the input object file.
Supporting this would probably be a pain given our current design, and
it seems like enough of an edge case that it's onot worth it.

Add the `--{no-}separate-cstring-literal-sections` option to emit
cstring literals into sections defined by their section name. This
allows for changes like https://github.com/swiftlang/swift/pull/84300
and https://github.com/swiftlang/swift/pull/84236 to actually have an
affect. The default behavior has not changed.

The reason this is useful is because strings in different sections might
have different access patterns at runtime. By splitting these strings
into separate sections, we may reduce the number of page faults during
startup. For example, the ObjC runtime accesses all strings in
`__objc_classname` before main.

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

Currently, our `safe` ICF mode only merges non-address-significant code,
leaving duplicate address-significant functions in the output. This
patch introduces `safe_thunks` ICF mode, which keeps a single master
copy of each function and replaces address-significant duplicates with
thunks that branch to the master copy.
Currently `--icf=safe_thunks` is only supported for `arm64`
architectures.

**Perf stats for a large binary:**
| ICF Option | Total Size | __text Size | __unwind_info | % total |

|-------------------|------------|-------------|---------------------|---------------------------|
| `--icf=none` | 91.738 MB | 55.220 MB | 1.424 MB | 0% |
| `--icf=safe` | 85.042 MB | 49.572 MB | 1.168 MB | 7.30% |
| `--icf=safe_thunks` | 84.650 MB | 49.219 MB | 1.143 MB | 7.72% |
| `--icf=all` | 82.060 MB | 48.726 MB | 1.111 MB | 10.55% |

So overall we can expect a `~0.45%` binary size reduction for a typical
large binary compared to the `--icf=safe` option.

**Runtime:**
Linking the above binary took ~10 seconds. Comparing the link
performance of --icf=safe_thunks vs --icf=safe, a ~2% slowdown was
observed.

This supersedes https://github.com/llvm/llvm-project/pull/87818 and
fixes https://github.com/llvm/llvm-project/issues/52767

When calculating arm64 thunks, we make a few assumptions that may not
hold when considering code sections outside of `__text`:

1. That a section needs thunks only if its size is larger than the
branch range.
2. That any calls into `__stubs` are necessarily forward jumps (that is,
the section with the jump is ordered before `__stubs`)

Sections like this exist in the wild, most prominently the
`__lcxx_overrides` section introduced in
https://github.com/llvm/llvm-project/pull/69498

This change:
- Ensures that if one section in `__TEXT` gets thunks, all of them do.
- Makes all code sections in `__TEXT` contiguous (and guaranteed to be
placed before `__stubs`)

Currently, when moving symbols from one `InputSection` to another (like
in ICF) we directly update the symbol's `isec`, `unwindEntry` and
`size`. By doing this we lose the original information. This information
will be needed in a future change. Since when moving symbols we always
set the symbol's `wasCoalesced` and `isec-> replacement`, we can just
use this info to conditionally get the information we need at access
time.

Previously, `makeSyntheticInputSection` would create a new
`ConcatInputSection` without setting `live` explicitly for it. Without
`-dead_strip` this would be OK since `live` would default to `true`.
However, with `-dead_strip`, `live` would default to false, and it would
remain set to `false`.
This hasn't resulted in any issues so far since no code paths that
exposed this issue were present.
However a recent change - ObjC relative method lists
(https://github.com/llvm/llvm-project/pull/86231) exposes this issue by
creating relocations to the `SyntheticInputSection`.
When these relocations are attempted to be written, this ends up with a
crash(assert), since the `SyntheticInputSection` they refer to is marked
as dead (`live` = `false`).

With this change, we set the correct behavior - `live` will always be
`true`. We add a test case that before this change would trigger an
assert in the linker.

The MachO format supports relative offsets for ObjC method lists. This
support is present already in ld64. With this change we implement this
support in lld also.

Relative method lists can be identified by a specific flag (0x80000000)
in the method list header. When this flag is present, the method list
will contain 32-bit relative offsets to the current Program Counter
(PC), instead of absolute pointers.
Additionally, when relative method lists are used, the offset to the
selector name will now be relative and point to the selector reference
(selref) instead of the name itself.