[flang][driver] Use __FLANG_VERISION__ in f18.cpp (nfc)

Just a minor improvement suggested in a post-commit review here:
https://reviews.llvm.org/D94422

flang/tools/f18/f18.cpp

@@ -389,8 +389,7 @@ void Link(std::vector<std::string> &liblist, std::vector<std::string> &objects,
 
 int printVersion() {
   llvm::errs() << "\nf18 compiler (under development), version "
-               << FLANG_VERSION_MAJOR << "." << FLANG_VERSION_MINOR << "."
-               << FLANG_VERSION_PATCHLEVEL << "\n";
+               << FLANG_VERSION_STRING << "\n";
   return exitStatus;
 }