[NFC] Refactor initialisation of Darwin malloc zone fields into separate...

[NFC] Refactor initialisation of Darwin malloc zone fields into separate function called `InitMallocZoneFields()` which is now called from `ReplaceSystemMalloc()`.

Summary:
The motivation here is to be able support initialisation of the
malloc zone on other code paths.

rdar://problem/45284065

Reviewers: kubamracek, george.karpenkov

Subscribers: llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D54037

llvm-svn: 345983