clang_Driver_getExternalActionsForCommand_v0 does not expand response files

[cwakamo]

clang_Driver_getExternalActionsForCommand_v0 exists as a function to get a list of actions that would be performed given a set of clang driver arguments. However, it does not currently expand response files, which is a difference between it and the real clang driver.

This means that response files get treated as inputs, I believe, and passed on to other actions. (For instance, a response file with -Wl,<arg> gets passed as-is to the linker, which causes it to fail because the linker doesn't understand -Wl.)

clang_Driver_getExternalActionsForCommand_v0 should be updated to expand response files.

[cwakamo]

Snagging this because it should be relatively straightforward.