Bastien Schatt authored Aug 18, 2015 and Brad King committed Aug 18, 2015

CMAKE_<LANG>_OUTPUT_EXTENSION tells us the proper extension for the
current toolchain.  Teach the ComputeObjectFilenames method to use
GetLanguageOutputExtension to look up the extension instead of
hard-coding ".obj".  This is already done in the code path for explicit
file names inside our call to GetObjectFileNameWithoutTarget.