Fix C++ support on recent DragonFly BSD releases

Fix C++ support on recent DragonFly BSD releases

Summary:
[ Copied from https://llvm.org/bugs/show_bug.cgi?id=25597 ]

Clang support for DragonFly BSD is lagging a bit, resulting in poor
support for c++.

DragonFlyBSD is unique in that it has two base compilers.  At the time
of the last Clang update for DragonFly, these compilers were GCC 4.4 and
GCC 4.7 (default).

With DragonFly Release 4.2, GCC 4.4 was replaced with GCC 5.0, partially
because the C++11 support of GCC 4.7 was incomplete.  The DragonFly
project will Release version 4.4 soon.

This patch updates the Clang driver to use libstdc++ from GCC 5.2 The
support for falling back to the alternate compiler was removed for two
reasons:

1) The last release to use GCC 4.7 is DF 4.0 which has already reached EOL
2) GCC 4.7 libstdc++ is insufficient for many "ports"

Therefore, I think it is reasonable that the development version of
clang expects GCC 5.2 to be in place and not try to fall back to another
compiler.

The attached patch will do this.  The Tools.cpp file was signficantly
modified to fix the linking which had been changed somewhere along the
line.  The rest of the changes should be self-explanatory.

Reviewers: joerg, rsmith, davide

Subscribers: jrmarino, davide, cfe-commits

Differential Revision: http://reviews.llvm.org/D15166

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@256467 91177308-0d34-0410-b5e6-96231b3b80d8