[llvm] r221960 - IR: Rewrite uniquing and creation of MDString

[Duncan P. N. Exon Smith]

> On 2014 Nov 18, at 10:06, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Unexposed, but not magic - it can be implemented in LLVM as it is in libc++. A few of these utilities are a bit of a pain to write, but they're generic enough not to need to be written very often. 

This is spiraling into a major project :).  I just want to pass in
a context by reference to prevent reference/pointer disagreement
in `MDString::get()` vs. `MDString::MDString()`, so I want some
API that is clear and correct.

How about I file a PR for this?  Your argument that `emplace()`
would be a good abstraction here is convincing, but encapsulating
`StringMap` is pretty orthogonal to my current work.

In the meantime, I think the perfect-forwarding I added in the
3-arg version of `StringMapEntry::Create()` is an incremental
improvement.