[ConstraintSystem] Introduce a fallback type for key path literals

This is a very first step in attempt to move some of the logic
from `simplifyKeyPathConstraint` to the inference. This type is
going to be used as an anchor to trigger capability inference.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -2097,6 +2097,10 @@ bool TypeVarBindingProducer::computeNext() {
   }
 
   if (newBindings.empty()) {
+    // If key path type had contextual types, let's not attempt fallback.
+    if (TypeVar->getImpl().isKeyPathType() && !ExploredTypes.empty())
+      return false;
+
     // Add defaultable constraints (if any).
     for (auto *constraint : DelayedDefaults) {
       if (constraint->getKind() == ConstraintKind::FallbackType) {

lib/Sema/CSGen.cpp

@@ -3884,6 +3884,13 @@ namespace {
 
       CS.addKeyPathConstraint(kpTy, root, value, componentTypeVars, locator);
 
+      // Add a fallback constraint so we have an anchor to use for
+      // capability inference when there is no context.
+      CS.addUnsolvedConstraint(Constraint::create(
+          CS, ConstraintKind::FallbackType, kpTy,
+          BoundGenericType::get(kpDecl, /*parent=*/Type(), {root, value}),
+          CS.getConstraintLocator(E, ConstraintLocator::FallbackType)));
+
       return kpTy;
     }