[flang] Avoid left shifts of negative signed values (#84786)

Shifting left a signed, negative value is an undefined behavior in C++.

This was detected by the undefined behavior sanitizer.

Author: kparzysz

flang/include/flang/Evaluate/integer.h

@@ -150,7 +150,10 @@ class Integer {
           }
         }
       } else {
-        INT signExtension{-(n < 0)};
+        // Avoid left shifts of negative signed values (that's an undefined
+        // behavior in C++).
+        auto signExtension{std::make_unsigned_t<INT>(n < 0)};
+        signExtension = ~signExtension + 1;
         static_assert(nBits >= partBits);
         if constexpr (nBits > partBits) {
           signExtension <<= nBits - partBits;
@@ -474,7 +477,12 @@ class Integer {
     SINT n = ToUInt<UINT>();
     constexpr std::size_t maxBits{CHAR_BIT * sizeof n};
     if constexpr (bits < maxBits) {
-      n |= -(n >> (bits - 1)) << bits;
+      // Avoid left shifts of negative signed values (that's an undefined
+      // behavior in C++).
+      auto u{std::make_unsigned_t<SINT>(ToUInt())};
+      u = (u >> (bits - 1)) << (bits - 1); // Get the sign bit only.
+      u = ~u + 1; // Negate top bits if not 0.
+      n |= static_cast<SINT>(u);
     }
     return n;
   }