[Type checker] Tolerate missing generic environments better.

Before matching witnesses, ensure that we have a valid signature, and be
more tolerant of null generic environments by using the
mapTypeIntoContext entry point meant to handle null.

We haven't managed to get a decent reproducer for this, but it's
happening often enough, and this change hardens the affected code
paths.

Should fix rdar://problem/37255982.

Author: DougGregor

include/swift/AST/Decl.h

@@ -2300,9 +2300,7 @@ class ValueDecl : public Decl {
   /// Set the interface type for the given value.
   void setInterfaceType(Type type);
 
-  bool hasValidSignature() const {
-    return hasInterfaceType() && !isBeingValidated();
-  }
+  bool hasValidSignature() const;
 
   /// isSettable - Determine whether references to this decl may appear
   /// on the left-hand side of an assignment or as the operand of a

lib/AST/Decl.cpp

@@ -1957,6 +1957,10 @@ void ValueDecl::setInterfaceType(Type type) {
   TypeAndAccess.setPointer(type);
 }
 
+bool ValueDecl::hasValidSignature() const {
+  return hasInterfaceType() && !isBeingValidated();
+}
+
 Optional<ObjCSelector> ValueDecl::getObjCRuntimeName() const {
   if (auto func = dyn_cast<AbstractFunctionDecl>(this))
     return func->getObjCSelector();

lib/Sema/ConstraintSystem.cpp

@@ -1074,7 +1074,7 @@ void ConstraintSystem::openGeneric(
 
   // Create the type variables for the generic parameters.
   for (auto gp : sig->getGenericParams()) {
-    auto contextTy = genericEnv->mapTypeIntoContext(gp);
+    auto contextTy = GenericEnvironment::mapTypeIntoContext(genericEnv, gp);
     if (auto *archetype = contextTy->getAs<ArchetypeType>())
       locatorPtr = getConstraintLocator(
           locator.withPathElement(LocatorPathElt(archetype)));

lib/Sema/TypeCheckProtocol.cpp

@@ -992,9 +992,15 @@ bool WitnessChecker::findBestWitness(
         continue;
       }
 
-      if (!witness->hasInterfaceType())
+      if (!witness->hasValidSignature()) {
         TC.validateDecl(witness);
 
+        if (!witness->hasValidSignature()) {
+          doNotDiagnoseMatches = true;
+          continue;
+        }
+      }
+
       auto match = matchWitness(TC, Proto, conformance, DC,
                                 requirement, witness);
       if (match.isViable()) {