ldc2 exception handling on macOS

[Peter Jacobs]

I am working on a reasonable large Dlang program that links to C 
and Fortran libraries.  With recent updates on macOS, I was 
having trouble linking to the Fortran libraries but found the 
-L-no_compact_unwind allowed me to get a build going, so long as 
I also had the optimisation flag -O set.  If I omitted the 
optimisation flag, the program would no longer catch exceptions.

I have made a small example of this behaviour below.  The 
transcript shows first the program built with optimisation on and 
catching the exception.  The second part shows the build without 
optimisation failing to catch the exception.

Am I just using the compiler incorrectly or is there a bug?

```
peterj at mini test-exception % ./run_test_exception.sh
Compile code
Run without throwing an exception
Success: mf=1
mf=1
Run and cause the throwing of an exception that should be caught
Caught expection message: oops
mf=2
peterj at mini test-exception % nvim run_test_exception.sh
peterj at mini test-exception % ./run_test_exception.sh
Compile code
Run without throwing an exception
Success: mf=1
mf=1
Run and cause the throwing of an exception that should be caught
rt/dwarfeh.d:369: uncaught exception reached top of stack
This might happen if you're missing a top level catch in your 
fiber or signal handler
test_exception.MyException at test_exception.d(17): oops
----------------
??:? object.Throwable.TraceInfo 
core.runtime.defaultTraceHandler(void*) [0x1047e2283]
??:? _Dmain [0x10478507b]
./run_test_exception.sh: line 9:  1155 Abort trap: 6           
./test_exception fail
peterj at mini test-exception %
```

```
# run_test_exception.sh
echo "Compile code"
# ldc2 -O -L-no_compact_unwind test_exception.d
ldc2 -L-no_compact_unwind test_exception.d
# ldc2 test_exception.d
echo "Run without throwing an exception"
./test_exception ok
echo "Run and cause the throwing of an exception that should be 
caught"
./test_exception fail
```

```
// test_exception.d

import std.stdio;

class MyException : Exception {
     @nogc
     this(string message, string file=__FILE__, size_t 
line=__LINE__,
          Throwable next=null)
     {
         super(message, file, line, next);
     }
}

string get_stuff(string incoming) {
     string outgoing = "stuff";
     if (incoming == "fail") {
         throw new MyException("oops");
     }
     return outgoing;
}

int main(string[] args)
{
     double mf = 0.0;
     try {
         mf = 1.0;
         string txt = get_stuff(args[1]);
         writefln("Success: mf=%g", mf);
     } catch (MyException e) {
         writefln("Caught expection message: %s", e.msg);
         mf = 2.0;
     }
     writefln("mf=%g", mf);
     return 0;
}
```

```
peterj at mini test-exception % ldc2 --version
LDC - the LLVM D compiler (1.38.0):
   based on DMD v2.108.1 and LLVM 18.1.5
   built with LDC - the LLVM D compiler (1.38.0)
   Default target: arm64-apple-darwin23.5.0
   Host CPU: apple-m1
   http://dlang.org - http://wiki.dlang.org/LDC
```