Fix libc++ vector<bool> data formatter (bug #32553)

Summary:
The iteration list through the available data formatters was undefined,
which meant that the vector<bool> formatter kicked in only in cases
where it happened to be queried before the general vector formatter. To
fix this, I merge the two data formatter entries into one, and select
which implementation to use in the factory function.

Reviewers: jasonmolenda, tberghammer, EricWF

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D31880

llvm-svn: 300047