[clang] [ubsan] Add -fsanitize-merge (and -fno-sanitize-merge) (PR #120464)

[Vitaly Buka via cfe-commits]

================
@@ -696,6 +697,17 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
   TrappingKinds &= Kinds;
   RecoverableKinds &= ~TrappingKinds;
 
+  // Parse -f(no-)?sanitize-nonmerged-handlers flags
+  SanitizerMask AlwaysMerge; // Empty
----------------
vitalybuka wrote:

```
```
  SanitizerMask MergeKinds =
      parseSanitizeArgs(D, Args, DiagnoseErrors, MergeDefault, {},
                        {}, options::OPT_fsanitize_merge_handlers_EQ,
                        options::OPT_fno_sanitize_merge_handlers_EQ);
  remove: MergeKinds |= AlwaysMerge; // No-op
  remove: MergeKinds &= ~NeverMerge; // No-op
```

https://github.com/llvm/llvm-project/pull/120464