[Sema] Handle PlaceholderTypes in associated type inference

Avoid attempting to infer an associatedtype as a type that contains
placeholders.

include/swift/AST/TypeMatcher.h

@@ -117,6 +117,12 @@ private:
 #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, don't look at it further, but proceed
-      // to find other matches.
-      if (secondType->hasError())
+      // 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())
         return true;
 
       // If the second type is a generic parameter of the witness, the match

validation-test/compiler_crashers_2_fixed/ac7123f2338b128.swift

@@ -1,5 +1,5 @@
 // {"signature":"swift::TypeChecker::typeCheckTarget(swift::constraints::SyntacticElementTarget&, swift::optionset::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::DiagnosticTransaction*)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 enum a
   protocol b {
     associatedtype c

validation-test/compiler_crashers_2_fixed/c8563a4a60b27cf3.swift

@@ -1,5 +1,5 @@
 // {"signature":"swift::CanTypeVisitor<swift::TypeMatcher<desugarSameTypeRequirement(swift::Requirement, swift::SourceLoc, llvm::SmallVectorImpl<swift::Requirement>&, llvm::SmallVectorImpl<swift::InverseRequirement>&, llvm::SmallVectorImpl<swift::rewriting::RequirementError>&)::Matcher>::MatchVisitor, bool, swift::Type, swift::Type>::visit(swift::CanType, swift::Type, swift::Type)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 protocol a {
   associatedtype b associatedtype c associatedtype d func e(b, c) -> d
 }