[IDE] Disable constraint system diagnostics during code completion

It seems disabling diagnstics doesn't introduce any regression.

rdar://problem/50679428

Author: rintaro

include/swift/AST/DiagnosticEngine.h

@@ -634,7 +634,7 @@ namespace swift {
     }
 
     /// Return all \c DiagnosticConsumers.
-    ArrayRef<DiagnosticConsumer *> getConsumers() {
+    ArrayRef<DiagnosticConsumer *> getConsumers() const {
       return Consumers;
     }
 

include/swift/AST/DiagnosticSuppression.h

@@ -37,6 +37,7 @@ class DiagnosticSuppression {
 public:
   explicit DiagnosticSuppression(DiagnosticEngine &diags);
   ~DiagnosticSuppression();
+  static bool isEnabled(const DiagnosticEngine &diags);
 };
 
 }

lib/AST/DiagnosticEngine.cpp

@@ -923,6 +923,10 @@ DiagnosticSuppression::~DiagnosticSuppression() {
     diags.addConsumer(*consumer);
 }
 
+bool DiagnosticSuppression::isEnabled(const DiagnosticEngine &diags) {
+  return diags.getConsumers().empty();
+}
+
 BufferIndirectlyCausingDiagnosticRAII::BufferIndirectlyCausingDiagnosticRAII(
     const SourceFile &SF)
     : Diags(SF.getASTContext().Diags) {

lib/Sema/TypeCheckConstraints.cpp

@@ -24,6 +24,7 @@
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/DiagnosticsParse.h"
+#include "swift/AST/DiagnosticSuppression.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Initializer.h"
 #include "swift/AST/NameLookup.h"
@@ -2175,7 +2176,8 @@ Type TypeChecker::typeCheckExpressionImpl(Expr *&expr, DeclContext *dc,
   // Construct a constraint system from this expression.
   ConstraintSystemOptions csOptions = ConstraintSystemFlags::AllowFixes;
 
-  if (options.contains(TypeCheckExprFlags::SuppressDiagnostics))
+  if (options.contains(TypeCheckExprFlags::SuppressDiagnostics) ||
+      DiagnosticSuppression::isEnabled(Diags))
     csOptions |= ConstraintSystemFlags::SuppressDiagnostics;
 
   if (options.contains(TypeCheckExprFlags::AllowUnresolvedTypeVariables))