[ASan] Change strdup interceptor to allow null input on Windows (#122803)

[These are the MS
Docs](https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/strdup-wcsdup-mbsdup?view=msvc-170)
regarding `strdup`, but they don't explicitly mention this. The SAL
annotations on `strdup` do, though, with the input parameter being
marked `_In_opt_z_`.

Author: zacklj89

compiler-rt/lib/asan/asan_interceptors.cpp

@@ -584,6 +584,9 @@ INTERCEPTOR(char *, strcpy, char *to, const char *from) {
 INTERCEPTOR(char*, strdup, const char *s) {
   void *ctx;
   ASAN_INTERCEPTOR_ENTER(ctx, strdup);
+  // Allowing null input is Windows-specific
+  if (SANITIZER_WINDOWS && UNLIKELY(!s))
+    return nullptr;
   if (UNLIKELY(!TryAsanInitFromRtl()))
     return internal_strdup(s);
   uptr length = internal_strlen(s);

compiler-rt/test/asan/TestCases/Windows/msvc/strdup_null_input.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cl_asan %s -Fe%t.exe
+// RUN: %run %t.exe | FileCheck %s
+
+// CHECK: Success
+
+#include <malloc.h>
+#include <stdio.h>
+#include <string.h>
+
+int main() {
+  // Null input is valid to strdup on Windows.
+  char *nullStr = _strdup(nullptr);
+  free(nullStr);
+  puts("Success");
+  return 0;
+}