[flang] Fix build errors for clang-10

[awarzynski]

Summary

Two NFCs that fix building with Clang-10. These are only wornings, but
since Flang sets -Werror, we need to get rid of this. As per
flang/README.md, Clang-10 is one of the officially supported compilers.

[awarzynski]

For reference:
@clementval: https://reviews.llvm.org/D87389

/home/flang/temp/llvm-project/flang/lib/Lower/OpenACC.cpp:102:32: error: unused variable 'argTy' [-Werror,-Wunused-variable]
    llvm::ArrayRef<mlir::Type> argTy;
                               ^
1 error generated.

@klausler https://reviews.llvm.org/D88672

/home/flang/temp/llvm-project/flang/runtime/io-stmt.h:165:1: error: 'ListDirectedStatementState' defined as a class template here but previously declared as a struct template; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags]
class ListDirectedStatementState<Direction::Input>
^
/home/flang/temp/llvm-project/flang/runtime/io-stmt.h:152:22: note: did you mean class here?
template <Direction> struct ListDirectedStatementState;
                     ^~~~~~
                     class
1 error generated.

Failing buildbot (stil staging, so no notifications are sent): http://lab.llvm.org:8014/#/builders/26

I'm not familiar with this code base, so suggestions are welcome.

[tskeith]

FYI, the io-stmt.h warning is fixed in https://reviews.llvm.org/D88711 (with them all declared as class). The other change is still needed.

[awarzynski]

@tskeith Thank you for the heads up. Updated accordingly and merged!

[clementval]

Thanks for fixing this.

flang/lib/Lower/OpenACC.cpp

static void genACC(Fortran::lower::AbstractConverter &converter,
   const auto &beginLoopDirective =
       std::get<Fortran::parser::AccBeginLoopDirective>(loopConstruct.t);
   const auto &loopDirective =
       std::get<Fortran::parser::AccLoopDirective>(beginLoopDirective.t);
 
   if (loopDirective.v == llvm::acc::ACCD_loop) {
     auto &firOpBuilder = converter.getFirOpBuilder();
     auto currentLocation = converter.getCurrentLocation();
-    llvm::ArrayRef<mlir::Type> argTy;
 
     // Add attribute extracted from clauses.
     const auto &accClauseList =
         std::get<Fortran::parser::AccClauseList>(beginLoopDirective.t);
 
     mlir::Value workerNum;
     mlir::Value vectorLength;
     mlir::Value gangNum;