swift234

[cmake] Link in the toolchain c++ headers after running LLVM cmake but before the LLVM build.

When we are building LLVM create symlinks to the c++ headers. We need to do this
before building LLVM since compiler-rt depends on being built with the just
built clang compiler. These are normally put into place during the cmake step of
LLVM's build when libcxx is in tree... but we are not building llvm with libcxx
in tree when we build swift. So we need to do cmake's work here.

rdar://26284108