[analyzer][NFC] CallDescription should own the qualified name parts

Previously, CallDescription simply referred to the qualified name parts
by `const char*` pointers.
In the future we might want to dynamically load and populate
`CallDescriptionMaps`, hence we will need the `CallDescriptions` to
actually **own** their qualified name parts.

Reviewed By: martong, xazax.hun

Differential Revision: https://reviews.llvm.org/D113593