Driver: Run 'clang' in C++ mode based on the name it was invoked by. We match

anything that ends with ++ or ++-FOO (e.g., c++, clang++, clang++-1.1) as being
a "C++ compiler".

This allows easy testing of the C++ compiler by 'ln -s clang clang++', or by 'cp
clang clang++'.

Based on patch by Roman Divacky.

llvm-svn: 86697