[flang] add attribute to trim runtime implementation establish call

[jeanPerier]

Summary

CFI allocatable attribute is needed so that the descriptor for the
result can be allcoated/deallocated.

flang/runtime/character.cpp

case CFI_type_char32_t:
     resultBytes = LenTrim(string.OffsetElement<const char32_t>(),
                       string.ElementBytes() >> 2)
         << 2;
     break;
   default:
     terminator.Crash(
         "TRIM: bad string type code %d", static_cast<int>(string.raw().type));
   }
-  result.Establish(string.type(), resultBytes, nullptr, 0);
+  result.Establish(string.type(), resultBytes, nullptr, 0, nullptr,
+      CFI_attribute_allocatable);
   RUNTIME_CHECK(terminator, result.Allocate(nullptr, nullptr) == CFI_SUCCESS);
   std::memcpy(result.OffsetElement(), string.OffsetElement(), resultBytes);
 }
 
 void RTNAME(CharacterMax)(Descriptor &accumulator, const Descriptor &x,
     const char *sourceFile, int sourceLine) {
   MaxMin<false>(accumulator, x, sourceFile, sourceLine);
 }