refactor -ccc-gcc-name code

refactor -ccc-gcc-name code

Put the logic for deciding the default name for gcc/g++
in the only place that actually cares about it.

This also pushes an ifdef out of the generic driver code
to a little further down, when the target is actually known.
Hopefully it can be changed into just a runtime check
in the future.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129212 91177308-0d34-0410-b5e6-96231b3b80d8