Revert removal of invalid interpolation check

Author: kitasuke

lib/Parse/ParseExpr.cpp

@@ -2064,6 +2064,15 @@ ParserResult<Expr> Parser::parseExprStringLiteral() {
 
     return makeParserResult(createStringLiteralExprFromSegment(Context, L, Segments.front(), Loc));
   }
+    
+  // We're not in a place where an interpolation would be valid.
+  if (!CurLocalContext) {
+    // Return an error, but include an empty InterpolatedStringLiteralExpr
+    // so that parseDeclPoundDiagnostic() can figure out why this string
+    // literal was bad.
+    return makeParserErrorResult(
+                                 new (Context) InterpolatedStringLiteralExpr(Loc, 0, 0, nullptr));
+  }
 
   unsigned LiteralCapacity = 0;
   unsigned InterpolationCount = 0;