[CSOptimizer] Don't attempt to walk into literals while analyzing operator chains

Most of them don't have any children but string interpolation does
and that marks whole chain as unsupported.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -500,7 +500,9 @@ static Type inferTypeOfArithmeticOperatorChain(DeclContext *dc, ASTNode node) {
             }
 
             literals.insert(defaultTy);
-            return Action::Continue(expr);
+            // String interpolation expressions have `TapExpr`
+            // as their children, no reason to walk them.
+            return Action::SkipChildren(expr);
           }
         }
       }

validation-test/Sema/type_checker_perf/fast/string_interpolations.swift.gyb

@@ -0,0 +1,26 @@
+// RUN: %scale-test --begin 1 --end 10 --step 1 --select NumLeafScopes %s -Xfrontend=-typecheck
+// REQUIRES: asserts,no_asan
+
+struct MyString {
+}
+
+extension MyString: ExpressibleByStringLiteral {
+  init(stringLiteral value: String) {}
+}
+
+func +(_: MyString, _: MyString) -> MyString {}
+
+func test(_: @autoclosure () -> String) {}
+func test(_: () -> String) {}
+func test(_: Character) {}
+
+func test(names: [String]) {
+  var resultString = ""
+  for name in names {
+    test("\(name)"
+%for i in range(0, N):
+      + "\(name)"
+%end
+      + "\(name)")
+  }
+}