[flang][runtime] Improve error message for incompatible MATMUL arguments

[klausler]

Summary

Print the full shapes of both argument when the dimensions that must
match do not do so.

flang/runtime/matmul.cpp

if constexpr (IS_ALLOCATING) {
     RUNTIME_CHECK(
         terminator, result.ElementBytes() == static_cast<std::size_t>(RKIND));
     RUNTIME_CHECK(terminator, result.GetDimension(0).Extent() == extent[0]);
     RUNTIME_CHECK(terminator,
         resRank == 1 || result.GetDimension(1).Extent() == extent[1]);
   }
   SubscriptValue n{x.GetDimension(xRank - 1).Extent()};
   if (n != y.GetDimension(0).Extent()) {
-    terminator.Crash("MATMUL: arrays do not conform (%jd != %jd)",
+    terminator.Crash("MATMUL: unacceptable operand shapes (%jdx%jd, %jdx%jd)",
+        static_cast<std::intmax_t>(x.GetDimension(0).Extent()),
         static_cast<std::intmax_t>(n),
-        static_cast<std::intmax_t>(y.GetDimension(0).Extent()));
+        static_cast<std::intmax_t>(y.GetDimension(0).Extent()),
+        static_cast<std::intmax_t>(y.GetDimension(1).Extent()));
   }
   using WriteResult =
       CppTypeFor<RCAT == TypeCategory::Logical ? TypeCategory::Integer : RCAT,
           RKIND>;
   if constexpr (RCAT != TypeCategory::Logical) {
     if (x.IsContiguous() && y.IsContiguous() &&
         (IS_ALLOCATING || result.IsContiguous())) {
       // Contiguous numeric matrices