[PING][PATCH, trunk, 4.9, 4.8] Fix PR57653, filename information discarded when using -imacros

[Peter Bergner]

I'd like to ping the following patch that fixes PR57653.  This did
bootstrap and regtest with no regressions on powerpc64-linux.

    https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01571.html

Is this ok for trunk, 4.9 and 4.8?

Peter