[MSVC] Workaround missing search path for sanitizer headers.

This is to fix build errors "Cannot open include file:
'sanitizer/asan_interface.h'" when building LLVM with MSVC and
LLVM_USE_SANITIZER=Address.

asan_interface.h is not available in MSVC's search path, instead it is
located under %VCToolsInstallDir%/crt/src/sanitizer.
This is an alternate solution to https://reviews.llvm.org/D118159, to
avoid adding all internal crt sources to the header search paths.

Tested with visual studio 2019 v16.9.6 and visual studio 2022 v17.0.5

Reviewed By: aaron.ballman, rnk

Differential Revision: https://reviews.llvm.org/D118624

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)