[flang] Add lowering for ASCII character constant

[clementval]

Summary

Lower character constant of KIND = 1 with the
createStringLiteral helper function.

This patch is part of the upstreaming effort from fir-dev branch.

[kiranchandramohan]

LGTM. Including this in D118993 would be better.

flang/lib/Lower/ConvertExpr.cpp

254

Nit: may be an assert message also.

[clementval]

Rebase + address review comments

flang/lib/Lower/ConvertExpr.cpp

if constexpr (TC == Fortran::common::TypeCategory::Integer) {
       TODO(getLoc(), "genval real constant");
     } else if constexpr (TC == Fortran::common::TypeCategory::Complex) {
       TODO(getLoc(), "genval complex constant");
     } else /*constexpr*/ {
       llvm_unreachable("unhandled constant");
     }
   }
 
+  /// Convert a ascii scalar literal CHARACTER to IR. (specialization)
+  ExtValue
+  genAsciiScalarLit(const Fortran::evaluate::Scalar<Fortran::evaluate::Type<
+                        Fortran::common::TypeCategory::Character, 1>> &value,
+                    int64_t len) {
+    assert(value.size() == static_cast<std::uint64_t>(len));
+    return fir::factory::createStringLiteral(builder, getLoc(), value);
+  }
+
   template <Fortran::common::TypeCategory TC, int KIND>
   ExtValue
   genval(const Fortran::evaluate::Constant<Fortran::evaluate::Type<TC, KIND>>
              &con) {
     if (con.Rank() > 0)
       TODO(getLoc(), "genval array constant");
     std::optional<Fortran::evaluate::Scalar<Fortran::evaluate::Type<TC, KIND>>>
         opt = con.GetScalarValue();
     assert(opt.has_value() && "constant has no value");
     if constexpr (TC == Fortran::common::TypeCategory::Character) {
-      TODO(getLoc(), "genval char constant");
+      if constexpr (KIND == 1)
+        return genAsciiScalarLit(opt.value(), con.LEN());
+      TODO(getLoc(), "genval for Character with KIND != 1");
     } else {
       return genScalarLit<TC, KIND>(opt.value());
     }
   }
 
   fir::ExtendedValue genval(
       const Fortran::evaluate::Constant<Fortran::evaluate::SomeDerived> &con) {
     TODO(getLoc(), "genval constant derived");