Reword ELAST warning

Summary:
GCC emits a pretty amusing warning when there are apostrophes in a #warning:
```warning: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wliteral-suffix]```

Reword the warning to avoid this, and be more consistent with other warnings in libc++.

Reviewers: danalbert

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7818

llvm-svn: 230298