Adds a way for tools to deduce the target config from a compiler name.

Adds `addTargetAndModeForProgramName`, a utility function that will add
appropriate `-target foo` and `--driver-mode=g++` tokens to a command
line for driver invocations of the form `a/b/foo-g++`. It is intended to
support tooling: for example, should a compilation database record some
invocation of `foo-g++` without these implicit flags, a Clang tool may
use this function to add them back.

Patch by Luke Zarko.

llvm-svn: 249391