[flang] Make interactive behaviour more obvious

[richard.barton.arm]

Summary

When flang is invoked with no files it waits for input on stdin. Make it
print a message saying this to prevent the user being surprised.

[DavidTruby]

LGTM after clang-format fixes

[richard.barton.arm]

Fix clang-format error.

flang/test/Driver/Inputs/hello.f90

+program hello
+  write (*,*), "hello world" 
+end program hello

flang/test/Driver/no_files.f90

+! RUN: %f18 < %S/Inputs/hello.f90 | FileCheck %s
+
+
+! CHECK: Enter Fortran source
+! CHECK: Use EOF character (^D) to end file
+
+! CHECK: Parse tree comprises {{.*}} objects and occupies {{.*}} total bytes
+! CHECK: PROGRAM hello
+! CHECK:  WRITE (*, *) "hello world"
+! CHECK: END PROGRAM hello

flang/tools/f18/f18.cpp

if (isPGF90) {
     }
 
     Fortran::parser::useHexadecimalEscapeSequences = true;
   }
 
   if (!anyFiles) {
     driver.measureTree = true;
     driver.dumpUnparse = true;
+    llvm::outs() << "Enter Fortran source\n"
+                 << "Use EOF character (^D) to end file\n";
     CompileFortran("-", options, driver, defaultKinds);
     return exitStatus;
   }
   for (const auto &path : fortranSources) {
     std::string relo{CompileFortran(path, options, driver, defaultKinds)};
     if (!driver.compileOnly && !relo.empty()) {
       objlist.push_back(relo);
     }