Support LLVM exception handling

[langston-barrett]

Update the CPG to better account for exceptions.

Some tasks:

• [ ] Add CallReturn nodes for exceptional return values (perhaps CallThrow?)
• [ ] Distinguish between regular and exceptional return in control flow graph
• [ ] Connect exceptional return values with landing pads
• [ ] Consider whether to add implicit exceptions for arithmetic, etc.

Migrated from internal (Gitlab) MATE issue number(s) 735