[Analyzer] Refactor begin and end symbol creation

[Analyzer] Refactor begin and end symbol creation

This patch refactors begin and end symbol creation by moving symbol
conjuration into the `create...` functions. This way the functions'
responsibilities are clearer and makes possible to add more functions
handling these symbols (e.g. functions for handling the container's
size) without code multiplication.

Differential Revision: https://reviews.llvm.org/D61136

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361141 91177308-0d34-0410-b5e6-96231b3b80d8