Add `//` before the banner displayed in `--print-ir-before/after-all`

[mehdi_amini]

Summary

This is making the output file (when the stream is a file) a valid MLIR
file.

[rriddle]

nit: Can you add a space between the / and the *?

[jpienaar]

Does it make it valid? Wouldn't you have the same function being redefined?

[mehdi_amini]

In D78604#1999852, @jpienaar wrote:

Does it make it valid? Wouldn't you have the same function being redefined?

In TensorFlow: we emit a new file on every invocation right now, and we print a full module. When I get dump for reproduction the only change I make is opening the file and deleting the first line :)

mlir/lib/Pass/IRPrinting.cpp

 void IRPrinterInstrumentation::runBeforePass(Pass *pass, Operation *op) {
   if (isHiddenPass(pass))
     return;
   // If the config asked to detect changes, record the current fingerprint.
   if (config->shouldPrintAfterOnlyOnChange())
     beforePassFingerPrints.try_emplace(pass, op);
 
   config->printBeforeIfEnabled(pass, op, [&](raw_ostream &out) {
-    out << formatv("*** IR Dump Before {0} ***", pass->getName());
+    out << formatv("// *** IR Dump Before {0} ***", pass->getName());
     printIR(op, config->shouldPrintAtModuleScope(), out, OpPrintingFlags());
     out << "\n\n";
   });
 }
 
 void IRPrinterInstrumentation::runAfterPass(Pass *pass, Operation *op) {
   if (isHiddenPass(pass))
     return;
   // If the config asked to detect changes, compare the current fingerprint with
   // the previous.
   if (config->shouldPrintAfterOnlyOnChange()) {
     auto fingerPrintIt = beforePassFingerPrints.find(pass);
     assert(fingerPrintIt != beforePassFingerPrints.end() &&
            "expected valid fingerprint");
     // If the fingerprints are the same, we don't print the IR.
     if (fingerPrintIt->second == OperationFingerPrint(op)) {
       beforePassFingerPrints.erase(fingerPrintIt);
       return;
     }
     beforePassFingerPrints.erase(fingerPrintIt);
   }
 
   config->printAfterIfEnabled(pass, op, [&](raw_ostream &out) {
-    out << formatv("*** IR Dump After {0} ***", pass->getName());
+    out << formatv("// *** IR Dump After {0} ***", pass->getName());
     printIR(op, config->shouldPrintAtModuleScope(), out, OpPrintingFlags());
     out << "\n\n";
   });
 }
 
 void IRPrinterInstrumentation::runAfterPassFailed(Pass *pass, Operation *op) {
   if (isAdaptorPass(pass))
     return;
   if (config->shouldPrintAfterOnlyOnChange())
     beforePassFingerPrints.erase(pass);
 
   config->printAfterIfEnabled(pass, op, [&](raw_ostream &out) {
-    out << formatv("*** IR Dump After {0} Failed ***", pass->getName());
+    out << formatv("// *** IR Dump After {0} Failed ***", pass->getName());
     printIR(op, config->shouldPrintAtModuleScope(), out,
             OpPrintingFlags().printGenericOpForm());
     out << "\n\n";
   });
 }
 
 //===----------------------------------------------------------------------===//
 // IRPrinterConfig