Revert "[Sema] Handle PlaceholderTypes in associated type inference"

This reverts commit aa9aa58.

Author: hamishknight

include/swift/AST/TypeMatcher.h

@@ -117,12 +117,6 @@ class TypeMatcher {
 #define SINGLETON_TYPE(SHORT_ID, ID) TRIVIAL_CASE(ID##Type)
 #include "swift/AST/TypeNodes.def"
 
-    bool visitPlaceholderType(CanPlaceholderType firstType, Type secondType,
-                              Type sugaredFirstType) {
-      // Placeholder types never match.
-      return mismatch(firstType.getPointer(), secondType, sugaredFirstType);
-    }
-
     bool visitUnresolvedType(CanUnresolvedType firstType, Type secondType,
                              Type sugaredFirstType) {
       // Unresolved types never match.

lib/Sema/AssociatedTypeInference.cpp

@@ -2317,9 +2317,9 @@ AssociatedTypeInference::getPotentialTypeWitnessesByMatchingTypes(ValueDecl *req
     /// Deduce associated types from dependent member types in the witness.
     bool mismatch(DependentMemberType *firstDepMember,
                   TypeBase *secondType, Type sugaredFirstType) {
-      // If the second type is an error or placeholder, don't look at it
-      // further, but proceed to find other matches.
-      if (secondType->hasError() || secondType->hasPlaceholder())
+      // If the second type is an error, don't look at it further, but proceed
+      // to find other matches.
+      if (secondType->hasError())
         return true;
 
       // If the second type is a generic parameter of the witness, the match