Remove support for Gentoo subversion-ebuild installed llvm-gcc libstdc++

Remove support for Gentoo subversion-ebuild installed llvm-gcc libstdc++
headers. As llvm-gcc is dead, and I have no idea if this ever really
worked, I think it's time for it to go. More importantly, it makes it
harder to generalize the include search logic. If someone really wants
these to work, they can set the CPLUS_INCLUDE_PATH environment variable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143836 91177308-0d34-0410-b5e6-96231b3b80d8