Add support in CMake to statically link the C++ standard library.

Add support in CMake to statically link the C++ standard library.

It is sometimes useful to have the C++ standard library linked into the
assembly when compiling clang, particularly when distributing a compiler
onto systems that don't have a copy of stdlibc++ or libc++ installed.

This functionality should work with either GCC or Clang as the host
compiler, though statically linking libc++ (as may be required for
licensing purposes) is only possible if the host compiler is Clang with
a copy of libc++ available.

Differential Revision: https://reviews.llvm.org/D65603

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368907 91177308-0d34-0410-b5e6-96231b3b80d8