Clang] Fix expansion of response files in -Wp after integrated-cc1 change

After rGb4a99a061f517e60985667e39519f60186cbb469, passing a response file such as -Wp,@a.rsp wasn't working anymore because .rsp expansion happens inside clang's main() function.

This patch adds response file expansion in the -cc1 tool.

Differential Revision: https://reviews.llvm.org/D73120