[CSFix] Add check to dont diagnose if has SubExpressionDiagnostics flag

Author: LucianoPAlmeida

lib/Sema/CSFix.cpp

@@ -1150,6 +1150,11 @@ bool RemoveUnnecessaryCoercion::attempt(ConstraintSystem &cs, Type fromType,
   if (ctx.LangOpts.DisableRedundantCoercionWarning)
     return false;
 
+  // Don't diagnose if this is coming from a CSDiag typecheck subExpr diagnostics.
+  // We can remove this once CSDiag is gone.
+  if (cs.Options.contains(ConstraintSystemFlags::SubExpressionDiagnostics))
+    return false;
+  
   auto last = locator.last();
   bool isExplicitCoercion =
       last && last->is<LocatorPathElt::ExplicitTypeCoercion>();
@@ -1168,10 +1173,6 @@ bool RemoveUnnecessaryCoercion::attempt(ConstraintSystem &cs, Type fromType,
   if (!fromType->isEqual(castType) && !castType->hasTypeVariable())
     return false;
 
-  // Fixed type was already applied to the subEpxr.
-  if (expr->getSubExpr()->getType())
-    return false;
-  
   auto toTypeRepr = expr->getCastTypeLoc().getTypeRepr();
 
   // Don't emit this diagnostic for Implicitly unwrapped optional types

test/expr/cast/as_coerce.swift

@@ -81,7 +81,7 @@ c3 as C4 // expected-error {{'C3' is not convertible to 'C4'; did you mean to us
 // <rdar://problem/19495142> Various incorrect diagnostics for explicit type conversions
 1 as Double as Float // expected-error{{cannot convert value of type 'Double' to type 'Float' in coercion}}
 1 as Int as String // expected-error{{cannot convert value of type 'Int' to type 'String' in coercion}}
-Double(1) as Double as String // expected-error{{cannot convert value of type 'Double' to type 'String' in coercion}} expected-warning {{redundant cast to 'Double' has no effect}} {{11-21=}}
+Double(1) as Double as String // expected-error{{cannot convert value of type 'Double' to type 'String' in coercion}}
 ["awd"] as [Int] // expected-error{{cannot convert value of type 'String' to expected element type 'Int'}}
 ([1, 2, 1.0], 1) as ([String], Int)
 // expected-error@-1 2 {{cannot convert value of type 'Int' to expected element type 'String'}}