Improve handling of clang's --config flag

[isbadawi]

Here is the documentation for this flag: https://clang.llvm.org/docs/UsersManual.html#configuration-files

Currently ccache handles it by hashing the contents of the config file, but this could lead to incorrect cache hits if the file happens to contain certain arguments like -fplugin= that would have been treated specially had they been specified directly on the command line instead of in the file. We could imagine treating the config file more like @-files instead, but there are some small differences to handle in the format.

(I guess the same is true for gcc's --specs flag which ccache handles the same way, but that format seems much more complex. I'm not super familiar with it. https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html)

As suggested in #850 (comment)