[flang] Make interactive behaviour more obvious

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.

Differential Revision: https://reviews.llvm.org/D84855

(cherry picked from commit dd5ea56)

flang/test/Driver/Inputs/hello.f90

@@ -0,0 +1,3 @@
+program hello
+  write (*,*), "hello world" 
+end program hello

flang/test/Driver/no_files.f90

@@ -0,0 +1,10 @@
+! 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

@@ -683,6 +683,8 @@ int main(int argc, char *const argv[]) {
   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;
   }