Revert "[flang] Avoid undefined behaviour when parsing format expressions (#147539)"

This reverts commit d0caf0d.

MathExtras.h is not found in some builds.

Author: DavidSpickett

flang/include/flang/Common/format.h

@@ -11,7 +11,6 @@
 
 #include "Fortran-consts.h"
 #include "enum-set.h"
-#include "llvm/Support/MathExtras.h"
 #include <cstring>
 
 // Define a FormatValidator class template to validate a format expression
@@ -215,18 +214,16 @@ template <typename CHAR> void FormatValidator<CHAR>::NextToken() {
   case '7':
   case '8':
   case '9': {
+    int64_t lastValue;
     const CHAR *lastCursor;
     integerValue_ = 0;
     bool overflow{false};
     do {
+      lastValue = integerValue_;
       lastCursor = cursor_;
-      if (LLVM_LIKELY(!overflow)) {
-        overflow = llvm::MulOverflow(
-            static_cast<int64_t>(10), integerValue_, integerValue_);
-      }
-      if (LLVM_LIKELY(!overflow)) {
-        overflow = llvm::AddOverflow(
-            integerValue_, static_cast<int64_t>(c - '0'), integerValue_);
+      integerValue_ = 10 * integerValue_ + c - '0';
+      if (lastValue > integerValue_) {
+        overflow = true;
       }
       c = NextChar();
     } while (c >= '0' && c <= '9');