Driver: do not strip file extensions when printing diagnostics.

Before: clang-3: error: no input files
After:  clang-3.3: error: no input files

This means that we'll also print clang.exe on windows, but I don't see a problem
with that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176788 91177308-0d34-0410-b5e6-96231b3b80d8