[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
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