[lldb] Process formatters in reverse-chronological order
If one reverts D66398 then the TestDataFormatterStdList does fail - as the C++
formatters are initialized in the opposite order. But the current state of
trunk does not mind the order for C++ formatters.
It is using now a single std::vector as suggested by Pavel Labath.
Differential Revision: https://reviews.llvm.org/D66654