[ASan] Introduce AllocatorOptions to configure allocator behavior.

Summary:
Reduce the dependency of allocator code on runtime flags. Instead,
pass a bunch of options that configure allocator behavior at
initialization or re-initialization. That would allow us to
cleaner modify allocator behavior during a program execution
when ASan is activated or de-activated.

Test Plan: regression test suite

Reviewers: kcc

Subscribers: llvm-commits, eugenis

Differential Revision: http://reviews.llvm.org/D6711

llvm-svn: 224605