diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
index f3317049524f3..1f044813bd74a 100644
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -39,6 +39,10 @@
 # define __has_builtin(x) 0
 #endif
 
+#ifndef __has_include
+# define __has_include(x) 0
+#endif
+
 // Only use __has_cpp_attribute in C++ mode. GCC defines __has_cpp_attribute in
 // C mode, but the :: in __has_cpp_attribute(scoped::attribute) is invalid.
 #ifndef LLVM_HAS_CPP_ATTRIBUTE
@@ -444,8 +448,21 @@
 /// Whether LLVM itself is built with AddressSanitizer instrumentation.
 #if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
 # define LLVM_ADDRESS_SANITIZER_BUILD 1
+#if __has_include(<sanitizer/asan_interface.h>)
 # include <sanitizer/asan_interface.h>
 #else
+// These declarations exist to support ASan with MSVC. If MSVC eventually ships
+// asan_interface.h in their headers, then we can remove this.
+#ifdef __cplusplus
+extern "C" {
+#endif
+void __asan_poison_memory_region(void const volatile *addr, size_t size);
+void __asan_unpoison_memory_region(void const volatile *addr, size_t size);
+#ifdef __cplusplus
+} // extern "C"
+#endif
+#endif
+#else
 # define LLVM_ADDRESS_SANITIZER_BUILD 0
 # define __asan_poison_memory_region(p, size)
 # define __asan_unpoison_memory_region(p, size)