Modify BaseFormatter to help static analysers

Modify BaseFormatter to help static analysers

Summary:
This decouples the format-string-parser from the storage of the arguments to be
formatted: a static analyser can now define a class which derives from
`BaseFormatter` with no storage and use `BaseFormatter::operator()` to verify
the syntax of the format string and determine which arguments will be
referenced.

This method of allowing overrides is resolved at compile time; the benchmarks
confirm that there is no run-time difference.

Reviewed By: yfeldblum

Differential Revision: D4507689

fbshipit-source-id: f109d81ae54dc074ac363720ef6e565520435d26