[sanitizer] Make __sanitizer::CheckFailed not public

Summary:
As far as I can tell, there is no reason why `__sanitizer::CheckFailed` should
be exported. Looking back in time, it was added with the FIXME with the
following by @timurrrr:

```
[*San/RTL] Fix minor breakage
Grumbling: this hasn't been caught by running 'make check-{a,l,t}san check-sanitizer'
```

I can't find any detail about the breakage, all tests seem to work for me, so
maybe Windows (@rnk?) or something I have no setup for.

The reason to make it private (past the FIXME) is that Scudo defines its own
(without callback) and I am trying to make the .so be loadable with the UBsan
one (that has its own public `CheckFailed`) with as little drama as possible.

Reviewers: eugenis, rnk

Reviewed By: eugenis, rnk

Subscribers: kubamracek, delcypher, #sanitizers, timurrrr, rnk, llvm-commits

Differential Revision: https://reviews.llvm.org/D52279

llvm-svn: 342651