This PR adds a statistics provider cache, which allows an individual
target to keep a rolling tally of it's total time and number of
invocations for a given summary provider. This information is then
available in statistics dump to help slow summary providers, and gleam
more into insight into LLDB's time use.
There is still the bug that empty lines seem to skip any following expressions
and it makes it harder to commend between all the comments. Let's make this
a normal test instead which is just slightly more verbose but can be properly
formatted.