[Sema] Assert that tap vars are in the current DeclContext (swiftlang#20303)

This assertion would have caught the last-minute bug in the string interpolation rework.

Author: beccadax

lib/Sema/CSGen.cpp

@@ -3086,6 +3086,11 @@ namespace {
     }
 
     Type visitTapExpr(TapExpr *expr) {
+      DeclContext *varDC = expr->getVar()->getDeclContext();
+      assert(varDC == CS.DC || (varDC && isa<AbstractClosureExpr>(varDC) &&
+              cast<AbstractClosureExpr>(varDC)->hasSingleExpressionBody()) &&
+             "TapExpr var should be in the same DeclContext we're checking it in!");
+      
       auto locator = CS.getConstraintLocator(expr);
       auto tv = CS.createTypeVariable(locator);