`CallDescriptions` deserve its own translation unit.
This patch simply moves the corresponding parts.
Also includes the `CallDescription.h` where it's necessary.
Reviewed By: martong, xazax.hun, Szelethus
Differential Revision: https://reviews.llvm.org/D113587
`PathSensitiveBughReport` has a function to mark a symbol as interesting but
it was not possible to clear this flag. This can be useful in some cases,
so the functionality is added.
Reviewed By: NoQ
Differential Revision: https://reviews.llvm.org/D105637