[clang] [Darwin][Driver] Avoid duplicate -lc++ with -fsanitize=fuzzer (PR #161304)

[Dan Blackwell via cfe-commits]

================
@@ -1609,7 +1609,12 @@ void DarwinClang::AddLinkRuntimeLibArgs(const ArgList &Args,
     if (Sanitize.needsFuzzer() && !Args.hasArg(options::OPT_dynamiclib)) {
       AddLinkSanitizerLibArgs(Args, CmdArgs, "fuzzer", /*shared=*/false);
 
-        // Libfuzzer is written in C++ and requires libcxx.
+      // Libfuzzer is written in C++ and requires libcxx.
+      // Since darwin::Linker::ConstructJob already adds -l++ for clang++
+      // by default (checked via ShouldLinkCXXStdlib), we only link if
+      // we are not invoked as CXX. This avoids duplicate library errors
+      // on Darwin.
+      if (!getDriver().CCCIsCXX())
----------------
DanBlackwell wrote:

It looks like there are some more conditions to the `ShouldLinkCXXStdlib` check: 
https://github.com/ndrewh/llvm-project/blob/4096bc5f104537a993db036b02d90ece37e2c6fd/clang/lib/Driver/ToolChain.cpp#L1529

```
!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs,
                      options::OPT_nostdlibxx);
```

Is it possible that this evaluates to false and then we end up failing to link libc++ because we assumed that we'd have it due to invoking clang++? Perhaps this would be better:

```
if (!CmdArgs.contains("-lc++"))  // pseudocode
  AddCXXStdlibLibArgs(Args, CmdArgs);
```

https://github.com/llvm/llvm-project/pull/161304