[flang] Add a "not yet implemented" message for large, constant arrays

[PeteSteinfeld]

Summary

Rather than crashing in llvm code, nip things in the bud.

[jeanPerier]

LGTM. Do you know which llvm::SmallVector is blowing up later ?

[PeteSteinfeld]

In D135321#3839475, @jeanPerier wrote:

LGTM. Do you know which llvm::SmallVector is blowing up later ?

It's the one that has mlir::Attribute as an argument. The object is inIntializer.

flang/lib/Lower/ConvertExpr.cpp

#define GENBIN(GenBinEvOp, GenBinTyCat, GenBinFirOp)                           \
   template <Fortran::common::TypeCategory TC, int KIND>
   ExtValue genArrayLit(
       const Fortran::evaluate::Constant<Fortran::evaluate::Type<TC, KIND>>
           &con) {
     mlir::Location loc = getLoc();
     mlir::IndexType idxTy = builder.getIndexType();
     Fortran::evaluate::ConstantSubscript size =
         Fortran::evaluate::GetSize(con.shape());
+    if (size > std::numeric_limits<std::uint32_t>::max())
+      // llvm::SmallVector has limited size
+      TODO(getLoc(), "Creation of very large array constants");
     fir::SequenceType::Shape shape(con.shape().begin(), con.shape().end());
     mlir::Type eleTy;
     if constexpr (TC == Fortran::common::TypeCategory::Character)
       eleTy = converter.genType(TC, KIND, {con.LEN()});
     else
       eleTy = converter.genType(TC, KIND);
     auto arrayTy = fir::SequenceType::get(shape, eleTy);
     mlir::Value array;