[fir] Fix FlangOptimizerTests link on Solaris

As reported in Issue #53690,
`tools/flang/unittests/Optimizer/FlangOptimizerTests` `FAIL`s to link on
Solaris:

  Undefined                       first referenced
   symbol                             in file
  _ZN3fir7runtimeL8getModelIcEEPFN4mlir4TypeEPNS2_11MLIRContextEEv lib/libFIRBuilder.a(Reduction.cpp.o)

which is `mlir::Type (*fir::runtime::getModel<char>())(mlir::MLIRContext*)`.

`clang++` warn's

  In file included from /var/llvm/llvm-14.0.0-rc1/rc1/llvm-project/flang/lib/Optimizer/Builder/Runtime/Reduction.cpp:14:
  /var/llvm/llvm-14.0.0-rc1/rc1/llvm-project/flang/include/flang/Optimizer/Builder/Runtime/RTBuilder.h:60:34: warning: function 'fir::runtime::getModel<char>' has internal linkage but is not defined [-Wundefined-internal]
  static constexpr TypeBuilderFunc getModel();
                                   ^
  /var/llvm/llvm-14.0.0-rc1/rc1/llvm-project/flang/include/flang/Optimizer/Builder/Runtime/RTBuilder.h:289:29: note: used here
        TypeBuilderFunc ret = getModel<RT>();
                              ^

Fixed by adding an explicit template instantiation for `getModel<char>`.  I
suppose this is necessary because on Solaris `char` is `signed`.

Tested on `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D119438