C++ exceptions

[magras]

Code randomization breaks exception handling:

int main() {
    try { throw 42; }
    catch (int) {}
}

$ SZ_CODE=1 SZ_LOWER=1 szcc++ exception.cpp

...
LandingPadInst not the first non-PHI instruction in the block.
  %7 = landingpad { i8*, i32 }
          catch i8* %.indirect1
in function stabilizer_main
LLVM ERROR: Broken function found, compilation aborted!
...

[magras]

There are two big problems known to me:

1. The landingpad instruction requires a constant argument. Load from a relocation table isn't a constant expression.

   I think this issue can be worked around by adding a new pass for the llvm backend, but IIUC this backends are platform specific.

2. Exception handling uses instruction pointer to unwind, while the relocated function of course has different ip.

   I see two options:

   1. After moving a function, modify corresponding tables used during unwind.
   2. Hook functions related to throwing exceptions to adjust ips in a call stack so that they point to the original functions.

   I expect a lot of rakes on both ways.

For now I'm proposing to fix the code generation, but keep all exception handling disabled, so any thrown exception would crash a program. I'll make a pr for that.

[magras]

Also small note for the future: to enable exception handling remove lowerinvoke pass.