[lldb] Add log indicating which kind of data formatter

The `formatter` logs include a function name, but these functions are mostly templates
and the template type parameter is not printed, which is useful context.

This change adds a new log which is printed upon entry of `FormatManager::Get`, which
shows the formatter context as either `format`, `summary`, or `synthetic`.

Differential Revision: https://reviews.llvm.org/D154128