Revert "[Wunsafe-buffer-usage] Fix false positive when const sized array is indexed by const evaluatable expressions (llvm#119340)"

This reverts commit 64c2156.
Causes asserts, see
llvm#119340 (comment)

(cherry picked from commit 7dd34ba)

Author: nico

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -968,11 +968,8 @@ AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
     return false;
   }
 
-  Expr::EvalResult EVResult;
-  if (Node.getIdx()->EvaluateAsInt(EVResult, Finder->getASTContext())) {
-    llvm::APSInt ArrIdx = EVResult.Val.getInt();
-    // FIXME: ArrIdx.isNegative() we could immediately emit an error as that's a
-    // bug
+  if (const auto *IdxLit = dyn_cast<IntegerLiteral>(Node.getIdx())) {
+    const APInt ArrIdx = IdxLit->getValue();
     if (ArrIdx.isNonNegative() && ArrIdx.getLimitedValue() < limit)
       return true;
   }

clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp

@@ -92,35 +92,3 @@ char access_strings() {
   c = array_string[5];
   return c;
 }
-
-struct T {
-  int array[10];
-};
-
-const int index = 1;
-
-constexpr int get_const(int x) {
-  if(x < 3)
-    return ++x;
-  else
-    return x + 5;
-};
-
-void array_indexed_const_expr(unsigned idx) {
-  // expected-note@+2 {{change type of 'arr' to 'std::array' to label it for hardening}}
-  // expected-warning@+1{{'arr' is an unsafe buffer that does not perform bounds checks}}
-  int arr[10];
-  arr[sizeof(int)] = 5;
-
-  int array[sizeof(T)];
-  array[sizeof(int)] = 5;
-  array[sizeof(T) -1 ] = 3;
-
-  int k = arr[6 & 5];
-  k = arr[2 << index];
-  k = arr[8 << index]; // expected-note {{used in buffer access here}}
-  k = arr[16 >> 1];
-  k = arr[get_const(index)];
-  k = arr[get_const(5)]; // expected-note {{used in buffer access here}}
-  k = arr[get_const(4)];
-}