swift234

Add -linker-path frontend option to choose the linker

In most Linux distributions, installing a clang package other than the
default unversioned one will not install a symlink from /usr/bin/clang++
-> /usr/bin/clang++-N-M, which can break builds with a not so great
diagnostic (a separate problem).

"ld" and "clang++" are hard-coded in the link job actions, so provide a
frontend flag, -linker-path, as a customization point for these.

rdar://problem/23537079