[PATCH] Allow for the use of StringRef command line options instead of cl::opt<std::string>

[Chandler Carruth]

Since Reid pointed me at this thread, I thought I would provide some
hopefully useful ideas.

First, a clarification: we have a large number of users that are parsing
command line options with LLVM used as a library. They have requirements to
not leak memory in addition to not having a good way to persist the strings
being parsed. I'm very opposed to any direction which assumes that the
parsed flags are memory managed externally to LLVM because it isn't true
today and would regress a large amount of customer code.

It feels like there is a reasonably simple, reasonably clear strategy here:

1) flags don't own their memory, and point at a global constant on
construction
2) parsing flags allocates memory for new values (preferably on
LLVMContext), and sets the flags to point at these new things
3) when shutting down an LLVMContext, we reset the flags to their global
constants

Without doing something to take ownership of the memory when parsing flags,
this change will break a very large number of users of LLVM, so I don't
think there is going to be a significantly simpler solution here.

The only thing seriously broken about the above is that users of
LLVMContext in multiple threads cannot parse commandline flags. That's
already true though, so I don't worry about *this* breaking existing users.
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