test/Driver: Check that @ arguments that aren't files are handled

This tests the bug fix in llvm r196620.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196621 91177308-0d34-0410-b5e6-96231b3b80d8

test/Driver/at_file_missing.c

+// Make sure that arguments that begin with @ are left as is in the argument
+// stream, and also that @file arguments continue to be processed.
+
+// RUN: echo "%s -D FOO" > %t.args
+// RUN: %clang -rpath @executable_path/../lib @%t.args -### 2>&1 | FileCheck %s
+// CHECK: "-D" "FOO"
+// CHECK: "-rpath" "@executable_path/../lib"