The cxx_compiler function should not blindly return clang++ as the C++...

The cxx_compiler function should not blindly return clang++ as the C++ compiler if $(CC) contains "clang".
Instead, it should perform a textual replacement of $(CC) from "clang" to "clang++".  The same is true
for "llvm-gcc" to "llvm-g++" and for "gcc" to "g++".  This way, we keep the path component of the $(CC)
passed in from the user and do not end up with a mixed toolchains with different paths.

Ditto for a newly added function called cxx_linker.

llvm-svn: 123451