Use clang --driver-mode instead of guessing c++ compiler path

Summary:
When building the LLDB test programs, if your CC is clang it actually isn't safe to make CXX a string replace of "clang -> clang++". This falls down on unix configurations if your compiler is clang-${version}.

A safer approach is to use the "--driver-mode=g++" option to tell clang to act like clang++.

Reviewers: tfiala, zturner, labath

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25753

llvm-svn: 284550