[msan] Add store_context_size= flag.

A new flag to control stack trace size for store event (in
track-origins=2 mode) independently of malloc_context_size.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@211896 91177308-0d34-0410-b5e6-96231b3b80d8