[ASan][NFC] Refactor redzone size calculation

Refactor redzone size calculation. This will simplify changing the
redzone size calculation in future.

Note that AddressSanitizer.cpp violates the latest LLVM style guide in
various ways due to capitalized function names. Only code related to the
change here was changed to adhere to the style guide.

No functional change intended.

Reviewed By: andreyknvl

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81367