Make sure 'prefix-clang++' is aliased to 'prefix-clang', not 'clang'.

When aliasing tools, rather than using the base TOOLEXENAME, we should
instead use the built tool's basename (for 'make') or the installed
tool's basename (for 'make install').

This should not cause any changes for anyone building unprefixed 'clang'
and 'clang++' tools.

Patch by Rick Foos!

llvm-svn: 165189