[fir] Fix FlangOptimizerTests link on Solaris

[ro]

Summary

As reported in Issue #53690, tools/flang/unittests/Optimizer/FlangOptimizerTests FAILs 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.

[clementval]

LGTM

flang/include/flang/Optimizer/Builder/Runtime/RTBuilder.h

 }
 template <>
 constexpr TypeBuilderFunc getModel<const char32_t *>() {
   return [](mlir::MLIRContext *context) -> mlir::Type {
     return fir::ReferenceType::get(mlir::IntegerType::get(context, 32));
   };
 }
 template <>
+constexpr TypeBuilderFunc getModel<char>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(context, 8 * sizeof(char));
+  };
+}
+template <>
 constexpr TypeBuilderFunc getModel<signed char>() {
   return [](mlir::MLIRContext *context) -> mlir::Type {
     return mlir::IntegerType::get(context, 8 * sizeof(signed char));
   };
 }
 template <>
 constexpr TypeBuilderFunc getModel<void *>() {
   return [](mlir::MLIRContext *context) -> mlir::Type {