[clang][bytecode] Check memcmp builtin for one-past-the-end pointers (#170097)

We can't read from those and will run into an assertion sooner or later.

Fixes #170031

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1921,6 +1921,10 @@ static bool interp__builtin_memcmp(InterpState &S, CodePtr OpPC,
   if (PtrA.isDummy() || PtrB.isDummy())
     return false;
 
+  if (!CheckRange(S, OpPC, PtrA, AK_Read) ||
+      !CheckRange(S, OpPC, PtrB, AK_Read))
+    return false;
+
   // Now, read both pointers to a buffer and compare those.
   BitcastBuffer BufferA(
       Bits(ASTCtx.getTypeSize(ElemTypeA) * PtrA.getNumElems()));

clang/test/AST/ByteCode/builtin-functions.cpp

@@ -1545,6 +1545,13 @@ namespace Memcmp {
 
   int unknown;
   void foo(void) { unknown *= __builtin_memcmp(0, 0, 2); }
+
+  constexpr int onepasttheend(char a) {
+    __builtin_memcmp(&a, &a + 1, 1); // both-note {{read of dereferenced one-past-the-end pointer}}
+    return 1;
+  }
+  static_assert(onepasttheend(10)); // both-error {{not an integral constant expression}} \
+                                    // both-note {{in call to}}
 }
 
 namespace Memchr {