[FrontendTool] Use a null diagconsumer for verify instance

Use a null diagnostic consumer in the deterministic verify instance. The
constraint system will skip work if there are no consumer in the
diagnostic engine. Use a null consumer to make sure the same amount of
work is done for two runs.

Author: cachemeifyoucan

lib/FrontendTool/FrontendTool.cpp

@@ -26,6 +26,7 @@
 #include "swift/AST/ASTDumper.h"
 #include "swift/AST/ASTMangler.h"
 #include "swift/AST/AvailabilityScope.h"
+#include "swift/AST/DiagnosticConsumer.h"
 #include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/FileSystem.h"
@@ -2100,6 +2101,11 @@ int swift::performFrontend(ArrayRef<const char *> Args,
     // Setup a verfication instance to run.
     std::unique_ptr<CompilerInstance> VerifyInstance =
         std::make_unique<CompilerInstance>();
+    // Add a null diagnostic consumer to the diagnostic engine so the
+    // compilation will exercise all the diagnose code path but not emitting
+    // anything.
+    NullDiagnosticConsumer DC;
+    VerifyInstance->getDiags().addConsumer(DC);
     std::string InstanceSetupError;
     // This should not fail because it passed already.
     (void)VerifyInstance->setup(Invocation, InstanceSetupError, Args);

test/Frontend/output_determinism_check.swift

@@ -37,7 +37,9 @@
 // PCM_OUTPUT: remark: produced matching output file '{{.*}}{{/|\\}}test.pcm'
 
 public var x = 1
-public func test() {}
+public func test() {
+  precondition(x == 1, "dummy check")
+}
 
 class A {
   var a = 0