patch: add readonly argument attribute

[Nick Lewycky]

The attached patch implements support for 'readonly' and 'readnone' on
individual pointer-typed attributes. The llvm memcpy/memmove intrinsics are
given readonly on their source pointers, and FunctionAttrs is updated to
label known functions with readonly/readnone where appropriate, as well as
deduce the attributes from function bodies. The semantics are described in
the patch to LangRef.

These attributes don't permit any new optimization of the function to which
they are applied. They do allow optimization to be applied in the caller of
the function, when the pointer has not otherwise escaped.

'readonly' can be used to allow store-load propagation and load-load
folding across calls. It requires knowing that the callee can't have the
pointer through some other means (we already have capture tracking). This
shows up a lot in of C++ code that takes a "const Foo &" argument.

'readnone' is really just along for the ride since I'm adding 'readonly'
anyways, but if you'd like a justification it means the pointer isn't
dereferenced but may be compared. Consider std::set<char*>, which requires
a comparison operator<(char *lhs, char *rhs); That function would have
'readnone' on lhs and rhs. Furthermore, a nocapture readnone argument is a
dead argument.

No optimizations are included in this patch, this is complicated enough
already. Another optimization this will help enable is better memcpy
elimination, since we'll be able to show that the users of the
memcpy-destination don't mutate it, and with nocapture they don't compare
the pointer value itself, and can thus be migrated over to the
memcpy-source (assuming the source isn't deallocated). This optimization is
also the reason behind the reimplementation of hasPath in
http://llvm-reviews.chandlerc.com/D996 .

Please review! Mostly I'd like approval that it's okay to add these new
attributes, the implementation isn't as simple as I'd like but the
refactoring needed to make it simpler are much too invasive for a drive-by
refactoring.

Nick
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