[GWP-ASan] Minor refactor of optional components.
In preparation for the inbuilt options parser, this is a minor refactor
of optional components including:
- Putting certain optional elements in the right header files, according to their function and their dependencies.
- Cleaning up some old and mostly-dead code.
- Moving some functions into anonymous namespaces to prevent symbol export.
Reviewed By: cryptoad, eugenis
Differential Revision: https://reviews.llvm.org/D94117