[flang][driver] Fix -fdefault* family bug

[arnamoy10]

Summary

This patch provides a fix for the fdefault-* family in f18

(Please consult D96344 for details)

[awarzynski]

This patch will allow us to share the tests for -fdefault-{double-8|integer-8|real-8} and -flarge-sizes (introduced in https://reviews.llvm.org/D96344) between f18 and flang-new. This is a very much welcomed change, thank you!

I don't think that we need any changes for -module though. f18 already supports -module-dir and there are no tests using -module that would require this change in the future. Please, could you revert it?

Ideally this patch would be accompanied with tests, but since D96344 is practically ready to land (and which contains tests for this), I suggest that we merge this as is. Could you mention e.g. in the commit message that tests will be added in a forthcoming patch? Thank you!

[arnamoy10]

Taken out -module related changes, updated the commit message as well.

[awarzynski]

LGTM

flang/tools/f18/f18.cpp

struct DriverOptions {
   bool dumpCookedChars{false};
   bool dumpUnparse{false};
   bool dumpUnparseWithSymbols{false};
   bool dumpParseTree{false};
   bool dumpPreFirTree{false};
   bool dumpSymbols{false};
   bool debugNoSemantics{false};
   bool debugModuleWriter{false};
+  bool defaultReal8{false};
   bool measureTree{false};
   bool unparseTypedExprsToF18_FC{false};
   std::vector<std::string> F18_FCArgs;
   const char *prefix{nullptr};
   bool getDefinition{false};
   GetDefinitionArgs getDefinitionArgs{0, 0, 0};
   bool getSymbolsSources{false};
   std::optional<bool> forcePreprocessing; // -cpp & -nocpp
if (arg.empty()) {
       auto eq{arg.find('=')};
       if (eq == std::string::npos) {
         predefinitions.emplace_back(arg.substr(2), "1");
       } else {
         predefinitions.emplace_back(arg.substr(2, eq - 2), arg.substr(eq + 1));
       }
     } else if (arg.substr(0, 2) == "-U") {
       predefinitions.emplace_back(arg.substr(2), std::optional<std::string>{});
-    } else if (arg == "-fdefault-double-8") {
-      defaultKinds.set_defaultRealKind(4);
     } else if (arg == "-r8" || arg == "-fdefault-real-8") {
+      driver.defaultReal8 = true;
       defaultKinds.set_defaultRealKind(8);
+      defaultKinds.set_doublePrecisionKind(16);
+    } else if (arg == "-fdefault-double-8") {
+      if (!driver.defaultReal8) {
+        // -fdefault-double-8 has to be used with -fdefault-real-8
+        // to be compatible with gfortran
+        llvm::errs() << "Use of `-fdefault-double-8` requires `-fdefault-real-8`\n";
+        return EXIT_FAILURE;
+      }
+      // For -fdefault-double-8 + -fdefault-real-8, only the size of
+      // DOUBLE PRECISION type changes, the size of default real type stays
+      // the same
+      // https://gcc.gnu.org/onlinedocs/gfortran/Fortran-Dialect-Options.html
+      defaultKinds.set_doublePrecisionKind(8);
     } else if (arg == "-i8" || arg == "-fdefault-integer-8") {
       defaultKinds.set_defaultIntegerKind(8);
       defaultKinds.set_subscriptIntegerKind(8);
       defaultKinds.set_sizeIntegerKind(8);
       if (isPGF90) {
         driver.F18_FCArgs.push_back("-i8");
       } else {
         driver.F18_FCArgs.push_back("-fdefault-integer-8");