Remove some unnecessary mapTypeIntoContext calls from CSGen. (#11743)

These helpers are all about trying to decide if a particular overload
is "favored", but the answer won't change if a local archetype is
actually the type we're looking for, so there's no advantage to using
contextual types over interface types. On top of that, the calls to
mapTypeIntoContext are forcing the deserialization of several generic
environments that would have otherwise been left untouched, resulting
in unnecessary calls to the generic signature builder.

This isn't a recent change, but we're seeing strain around the generic
signature builder and complex constraints in the standard library
anyway, so we'll take wins where we can get them.

Discovered as part of the performance investigation in
rdar://problem/33984386.

Author: jrose-apple

lib/Sema/CSGen.cpp

@@ -644,8 +644,6 @@ namespace {
         auto overloadChoice = favoredConstraints[0]->getOverloadChoice();
         auto overloadType = overloadChoice.getDecl()->getInterfaceType();
         auto resultType = overloadType->getAs<AnyFunctionType>()->getResult();
-        resultType = overloadChoice.getDecl()->getInnermostDeclContext()
-            ->mapTypeIntoContext(resultType);
         CS.setFavoredType(expr, resultType.getPointer());
       }
 
@@ -731,10 +729,8 @@ namespace {
         fnTy = fnTy->getResult()->castTo<AnyFunctionType>();
       }
 
-      Type paramTy = value->getInnermostDeclContext()
-          ->mapTypeIntoContext(fnTy->getInput());
-      auto resultTy = value->getInnermostDeclContext()
-          ->mapTypeIntoContext(fnTy->getResult());
+      Type paramTy = fnTy->getInput();
+      auto resultTy = fnTy->getResult();
       auto contextualTy = CS.getContextualType(expr);
 
       return isFavoredParamAndArg(
@@ -808,9 +804,6 @@ namespace {
           }
         }
         Type paramTy = fnTy->getInput();
-        paramTy = value->getInnermostDeclContext()
-            ->mapTypeIntoContext(paramTy);
-        
         return favoredTy->isEqual(paramTy);
       };
 
@@ -895,17 +888,15 @@ namespace {
         fnTy = fnTy->getResult()->castTo<AnyFunctionType>();
       }
 
-      Type paramTy = value->getInnermostDeclContext()
-          ->mapTypeIntoContext(fnTy->getInput());
+      Type paramTy = fnTy->getInput();
       auto paramTupleTy = paramTy->getAs<TupleType>();
       if (!paramTupleTy || paramTupleTy->getNumElements() != 2)
         return false;
 
       auto firstParamTy = paramTupleTy->getElement(0).getType();
       auto secondParamTy = paramTupleTy->getElement(1).getType();
 
-      auto resultTy = value->getInnermostDeclContext()
-          ->mapTypeIntoContext(fnTy->getResult());
+      auto resultTy = fnTy->getResult();
       auto contextualTy = CS.getContextualType(expr);
 
       return