[clang] Add option -fstdlib-hardening= (PR #78763)

[Louis Dionne via cfe-commits]

================
@@ -851,6 +851,28 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
                       Twine(getClangFullCPPVersion()) + "\"");
 
   // Initialize language-specific preprocessor defines.
+  if (LangOpts.getStdlibHardeningMode()) {
+    const char *StdlibHardeningStr;
+
+    switch (LangOpts.getStdlibHardeningMode()) {
+    case clang::LangOptions::STDLIB_HARDENING_MODE_NOT_SPECIFIED:
+      llvm_unreachable("Unexpected libc++ hardening mode value");
+    case clang::LangOptions::STDLIB_HARDENING_MODE_NONE:
+      StdlibHardeningStr = "_STDLIB_HARDENING_MODE_NONE";
----------------
ldionne wrote:

The macros used by libc++ are `_LIBCPP_HARDENING_MODE`, `_LIBCPP_HARDENING_MODE_NONE` & al. While the naming of the clang option should use the "any standard library" terminology, the actual macros we set must be the libc++ ones. If libstdc++ had something similar in the future, we would switch to:

```c++
if (LangOpts.getStdlibHardeningMode()) {
  if (stdlib is libc++) {
    set-macros-for-libcxx;
  } else {
    set-macros-for-libstdcxx;
  }
}
```

https://github.com/llvm/llvm-project/pull/78763