[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