compiler_attributes.h: add VPX_NO_UNSIGNED_SHIFT_CHECK

and use it on MD5Transform(); this behavior is well defined and is only
a warning with -fsanitize=integer, not -fsanitize=undefined.

quiets warnings of the form:
md5_utils.c:163:3: runtime error: left shift of 143704723 by 7 places
cannot be represented in type 'unsigned int'

Bug: b/229626362
Change-Id: I60a384b2c2556f5ce71ad8ebce050329aba0b4e4

md5_utils.c

@@ -151,8 +151,8 @@ void MD5Final(md5byte digest[16], struct MD5Context *ctx) {
  * reflect the addition of 16 longwords of new data.  MD5Update blocks
  * the data and converts bytes into longwords for this routine.
  */
-VPX_NO_UNSIGNED_OVERFLOW_CHECK void MD5Transform(UWORD32 buf[4],
-                                                 UWORD32 const in[16]) {
+VPX_NO_UNSIGNED_OVERFLOW_CHECK VPX_NO_UNSIGNED_SHIFT_CHECK void MD5Transform(
+    UWORD32 buf[4], UWORD32 const in[16]) {
   UWORD32 a, b, c, d;
 
   a = buf[0];

vpx_ports/compiler_attributes.h

@@ -29,13 +29,23 @@
 #endif  // __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
 
 #if defined(__clang__) && __has_attribute(no_sanitize)
+// Both of these have defined behavior and are used in certain operations or
+// optimizations thereof. There are cases where an overflow may be unintended,
+// however, so use of these attributes should be done with care.
 #define VPX_NO_UNSIGNED_OVERFLOW_CHECK \
   __attribute__((no_sanitize("unsigned-integer-overflow")))
-#endif
+#if __clang_major__ >= 12
+#define VPX_NO_UNSIGNED_SHIFT_CHECK \
+  __attribute__((no_sanitize("unsigned-shift-base")))
+#endif  // __clang__ >= 12
+#endif  // __clang__
 
 #ifndef VPX_NO_UNSIGNED_OVERFLOW_CHECK
 #define VPX_NO_UNSIGNED_OVERFLOW_CHECK
 #endif
+#ifndef VPX_NO_UNSIGNED_SHIFT_CHECK
+#define VPX_NO_UNSIGNED_SHIFT_CHECK
+#endif
 
 //------------------------------------------------------------------------------
 // Variable attributes.