Merging r276232:

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r276232 | gbiv | 2016-07-20 20:28:13 -0700 (Wed, 20 Jul 2016) | 13 lines

[Sema] Fix PR28623.

In atomic builtins, we assumed that the LValue conversion on the first
argument would succeed. So, we would crash given code like:

```
void ovl(char);
void ovl(int);
__atomic_store_n(ovl, 0, 0);
```

This patch makes us not assume that said conversion is successful. :)

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llvm-svn: 276489