Add llvm::join_items to StringExtras.

llvm::join_items is similar to llvm::join, which produces a string
by concatenating a sequence of values together separated by a
given separator.  But it differs in that the arguments to
llvm::join() are same-type members of a container, whereas the
arguments to llvm::join_items are arbitrary types passed into
a variadic template.  The only requirement on parameters to
llvm::join_items (including for the separator themselves) is
that they be implicitly convertible to std::string or have
an overload of std::string::operator+

Differential Revision: https://reviews.llvm.org/D24880

llvm-svn: 282502