[Sema] Add null checks in ConstraintSystem::bindOverloadType to prevent a crash during indexing.

locator->getAnchor() may return a null Expr*, but was being passed directly to
isa<KeyPathExpr>(..) without a null check in two places.

Resovles rdar://problem/63558609

Author: Nathan Hawes

lib/Sema/ConstraintSystem.cpp

@@ -2062,7 +2062,7 @@ void ConstraintSystem::bindOverloadType(
 
     // If this is used inside of the keypath expression, we need to make
     // sure that argument is Hashable.
-    if (isa<KeyPathExpr>(locator->getAnchor()))
+    if (llvm::isa_and_nonnull<KeyPathExpr>(locator->getAnchor()))
       verifyThatArgumentIsHashable(0, argType, locator);
 
     // The resolved decl is for subscript(dynamicMember:), however the original
@@ -2184,7 +2184,7 @@ void ConstraintSystem::bindOverloadType(
       addConstraint(ConstraintKind::Equal, memberTy, leafTy, keyPathLoc);
     }
 
-    if (isa<KeyPathExpr>(locator->getAnchor()))
+    if (llvm::isa_and_nonnull<KeyPathExpr>(locator->getAnchor()))
       verifyThatArgumentIsHashable(0, keyPathTy, locator);
 
     // The resolved decl is for subscript(dynamicMember:), however the