[Type checker] Remove the now-unused InterpolationArgument locator element.

NFC; the prior commit eliminated all uses of this locator path element.

(cherry picked from commit 4127f70)

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -4102,22 +4102,7 @@ findCalleeDeclRef(ConstraintSystem &cs, const Solution &solution,
 
     unsigned newFlags = locator->getSummaryFlags();
 
-    // If we have an interpolation argument, dig out the constructor if we
-    // can.
-    // FIXME: This representation is actually quite awful
-    if (newPath.size() == 1 &&
-        newPath[0].getKind() == ConstraintLocator::InterpolationArgument) {
-      newPath.push_back(ConstraintLocator::ConstructorMember);
-
-      locator = cs.getConstraintLocator(locator->getAnchor(), newPath, newFlags);
-      auto known = solution.overloadChoices.find(locator);
-      if (known != solution.overloadChoices.end()) {
-        auto &choice = known->second.choice;
-        if (choice.getKind() == OverloadChoiceKind::Decl)
-          return cast<AbstractFunctionDecl>(choice.getDecl());
-      }
-      return nullptr;
-    } else if (isSubscript) {
+    if (isSubscript) {
       newPath.push_back(ConstraintLocator::SubscriptMember);
     } else {
       newPath.push_back(ConstraintLocator::ApplyFunction);

lib/Sema/CSDiag.cpp

@@ -222,22 +222,6 @@ void constraints::simplifyLocator(Expr *&anchor,
       }
       break;
 
-    case ConstraintLocator::InterpolationArgument:
-      if (auto interp = dyn_cast<InterpolatedStringLiteralExpr>(anchor)) {
-        unsigned index = path[0].getValue();
-        if (index < interp->getSegments().size()) {
-          // No additional target locator information.
-          // FIXME: Dig out the constructor we're trying to call?
-          targetAnchor = nullptr;
-          targetPath.clear();
-
-          anchor = interp->getSegments()[index];
-          path = path.slice(1);
-          continue;
-        }
-      }
-      break;
-
     case ConstraintLocator::SubscriptIndex:
       if (auto subscript = dyn_cast<SubscriptExpr>(anchor)) {
         targetAnchor = subscript->getBase();

lib/Sema/CSSolver.cpp

@@ -2465,17 +2465,6 @@ bool ConstraintSystem::solveSimplified(
         if (*restriction == ConversionRestrictionKind::TupleToTuple)
           break;
       }
-
-      // Or, if we see a conversion successfully applied to a string
-      // interpolation argument, we're done.
-      // FIXME: Probably should be more general, as mentioned above.
-      if (auto locator = disjunction->getLocator()) {
-        if (!locator->getPath().empty() &&
-            locator->getPath().back().getKind()
-              == ConstraintLocator::InterpolationArgument &&
-            constraint->getKind() == ConstraintKind::Conversion)
-          break;
-      }
     }
 
     if (TC.getLangOpts().DebugConstraintSolver) {

lib/Sema/ConstraintLocator.cpp

@@ -70,7 +70,6 @@ void ConstraintLocator::Profile(llvm::FoldingSetNodeID &id, Expr *anchor,
     case ScalarToTuple:
     case Load:
     case GenericArgument:
-    case InterpolationArgument:
     case NamedTupleElement:
     case TupleElement:
     case ApplyArgToParam:
@@ -164,10 +163,6 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) {
       out << "instance type";
       break;
 
-    case InterpolationArgument:
-      out << "interpolation argument #" << llvm::utostr(elt.getValue());
-      break;
-
     case Load:
       out << "load";
       break;

lib/Sema/ConstraintLocator.h

@@ -91,8 +91,6 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     SubscriptIndex,
     /// \brief The result of a subscript expression.
     SubscriptResult,
-    /// \brief An argument to string interpolation.
-    InterpolationArgument,
     /// \brief The lookup for a constructor member.
     ConstructorMember,
     /// \brief Rvalue adjustment.
@@ -155,7 +153,6 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       return 0;
 
     case GenericArgument:
-    case InterpolationArgument:
     case NamedTupleElement:
     case TupleElement:
       return 1;
@@ -204,7 +201,6 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     case Archetype:
     case AssociatedType:
     case GenericArgument:
-    case InterpolationArgument:
     case NamedTupleElement:
     case TupleElement:
     case Requirement:
@@ -335,12 +331,6 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       return PathElement(GenericArgument, position);
     }
 
-    /// \brief Retrieve a path element for an argument to string
-    /// interpolation.
-    static PathElement getInterpolationArgument(unsigned position) {
-      return PathElement(InterpolationArgument, position);
-    }
-
     /// \brief Retrieve the kind of path element.
     PathElementKind getKind() const {
       switch (static_cast<StoredKind>(storedKind)) {