Fix _NSCFBoolean data formatter.

In r265181 the test for the NSCFBoolean data formatter was removed.
Later, in r279353 and r279446 a new implementation was provided for the
formatter, which I believe never worked (and this wasn't caught because
the test was never re-enabled).

This commit fixes the bug and re-enables the old test case.

Differential revision: https://reviews.llvm.org/D47014

llvm-svn: 332700