[clang] [codegen] Fix crash in codegen caused by pointer calculation overflow (PR #115791)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

CodeGen should not produce errors for overflows which do not occur at compile-time according to the language semantics.  This is happening at runtime, so we need to just eat the error; we can emit an unconditional call to the ubsan error handler.  We are not allowed to reject valid programs.

Ideally Sema should produce a warning so users aren't caught off-guard.

https://github.com/llvm/llvm-project/pull/115791